HARRY E. MORTON (University of Pennsylvania, Philadelphia, Pa.) : I propose first to direct a few questions to Edward. I t is a great problem to do serologic testing when PPLO cannot be made to grow in liquid media or if they cannot be grown in sufficient quantities to produce an antigen. Can the sensitivity of your growth inhibition test be used for serologic indentification of strains of PPLO that will grow only on a solid medium? Would this be a reliable method, and how would it compare with the complement fixation and hemagglu tinat ion reactions? D. G. FF. EDWARD (The Wellcome Research Laboratories, Beckenham, Kent, England) : I use this test more and more for serologic investigations because it eliminates the difficulty in preparing antigen, although it has the disadvantage of being more expensive with serum. There is much still to be learned about the test, which has been used only to examine a few species of the group. It was used extensively by me for human genital strains from which it is notoriously difficult to obtain good agglutinating suspensions. With the inhibition of growth test more than 90 strains were identified serologically with no technical difficulty. I n the serologic identification of strains the agglutination test has the disadvantage that there are minor differences between strains, so that one speciesspecific antiserum may not agglutinate all strains. For Mycoplasma hominis the inhibition of growth test had the advantage that it was less specific, and one serum inhibited the growth of all strains of the species. Subsequent experience has shown that this may not be true for every species, and there is a suggestion that the inhibition of growth test may sometimes be more specific than the agglutination test. I n preparing antisera in rabbits, growth-inhibiting antibodies do not develop at the same time as do the agglutinating antibodies. Sometimes they appear first; more often the rabbit has to be inoculated repeatedly until they develop. I should be interested to learn whether inhibiting antibodies can be demonstrated in animals recovering from natural infection. I notice that many workers prepare their antigens for inoculating rabbits in media enriched with horse serum. I consider that there is a great danger in doing this, owing to the risk of producing antibodies to horse serum. Before starting this type of work I consulted serologists and they told me that the results would always be open to criticism if horse serum were used. I have, therefore, always enriched my media with rabbit serum. Originally, with strains that grew only poorly in rabbit serum, I spent weeks and months adapting them to grow in rabbit serum media. Eventually I discovered that these strains would grow equally well in a rabbit serum medium as they did in a horse serum medium if cholesterol was added. As it is easy to grow a strain in a medium from which horse serum is eliminated, I strongly advocate that this should be done in order to avoid the criticism that the results may be vitiated by the presence of antibodies to horse serum. In regard to the papers on tissue culture, I can report that I examined ColI mention a general point on the preparation of antisera in rabbits.

D. G. FP. EDWARD (Wellcome Research Laboratories, Beckenham, Kent, England): Ever since the first isolation of organisms of the pleuropneumonia group from the human genital tract, great interest has been taken in their relationship to infection. There would seem to be little doubt that the organisms can be pathogenic for man. Their pathogenicity would appear to be similar to that of Escherichia coli, which is a normal inhabitant of the gut, yet can produce a pyogenic infection if it gains access to other parts of the body. There seems little doubt that organisms of the pleuropneumonia group can cause puerperal fever, salpingitis, and abscesses in the neighborhood of the genital tract. However, in regard to pathogenicity the main question a t issue is whether the organisms are a cause of nongonococcal urethritis. This disease is of great importance and its etiology remains uncertain. Since these organisms are present in a proportion of cases it is tempting to suggest that they cause the infection. Many investigators have studied the relationship of the organisms to nongonococcal urethritis; in various surveys the proportion of positive isolations from cases has varied, but has never exceeded 50 per cent. On the other hand, similar organisms have been isolated from healthy individuals examined as controls. There are differences of opinion as to what constitutes a control group, and doubts have been expressed as to whether the genital tracts of all the controls were completely healthy. Klieneberger-Nobel has examined boys before puberty, but I regard as a control group men from the general population who are not attending a venereal disease clinic. I consider that it is highly significant that a high proportion of these men have Mycoplasma hominis in the urethra. On the other hand the organism can be isolated only from one half or less of the cases of nongonococcal urethritis, although it is not a difficult organism to isolate. There is no difference, clinically or in response to treatment, between cases with positive isolations and those in whom the organism cannot be demonstrated. These findings suggest strongly that M . hominis is not the cause of nongonococcal urethritis. It would appear more likely that it is a commensal, although it is impossible to exclude the possibility that it may produce some ill effects when it gains access to the urethra. It would seem that little can be gained from a repetition of the many surveys already carried out in which the organisms have been sought in diseased and healthy individuals. If the question of pathogenicity is to be decided a new approach must be made. E. A. FREUNDT (Statens Seruminstitut, Copenhagen, Denmark) : I do not think that the different incidence of PPLO in the male urethra found by KlienebergerNobel and by me can be explained by different methods of procuring specimens for cultivation. Actually, the technique I used would not seem to differ essentially from that of Klieneberger-Nobel. My specimens were not taken merely from the “external surface” of the urethral meatus, as supposed by Klieneberger-Nobel, but from the anterior urethra by means of cotton-wool swabs. These were inserted as deep as possible into the urethra after cleaning the meatus with saline. Moreover, I don’t think that the microbial flora of the