Although phage-typing of some pathogenic microbes-e.g., Escherichia coli and the genera Salmonella, Staphylococcus, and Streptomyces-is in routine use, little progress has yet been made in the phage-typing of mycobacteria, chiefly because mycobacteriophages possess a polyvalence that prevents the reliable identification of individual species of mycobacterium. Furthermore, laboratories cannot compare their results because they employ different methods and culture media and also because the phages used cannot be accurately identified. It has been shown that internationally reproducible results can be obtained for M. tuberculosis by selecting suitable mycobacteriophages and sensitive culture media, and by using a standardized technique of phage-typing. Under these conditions, phage lysis may be utilized to classify strains of M. tuberculosis.