A study was made in dynamics on the number of spleen cells producing antibodies following immunization of mice with erythrocytes. It was found that shortly after immunization the number of these cells was extremely small and subsequently grew rapidly. In animals given injections of 6-mercaptopurine or 6-thioguanine during immunization the number of antibody forming cells reduced considerably. Upon use of 6-thioguanine this effect was the most pronounced. The findings obtained are considered in the light of the clonal-selection theory.
