Sixty-eight substituted azaindolizines were tested for their activity on Lactobacillus casei and Streptococcus faecalis. These azaindolizines could be divided into three classes according to the pattern of growth inhibition, substituent present, and the position of the substituent in azaindolizine molecule. Compounds in Group 1 and 2 which have sulfurcontaining substituents and halogen groups, respectively, were not affected appreciably by folic acid, leucovorin, thymine, or purines, while Group 2 compounds which have amino or closely related groups were antagonized by folic acid, leucovorin, and thymine but not by purines.