The effects of 5-day courses of orally administered cephalexin, clindamycin and erythromycin on the Gram-negative, aerobic faecal flora of healthy adults were examined. The concentration of cephalexin reached in the intestine was high enough to cause the emergence of resistant Gram-negative bacteria; organisms belonging to the genera Enterobacter, Citrobacter and Pseudomonas increased to easily detectable levels. The faecal concentration of erythromycin was high and caused a severe reduction of the coliform flora. Clindamycin administration resulted in a considerable increase in the coliform count; the increase in the proportion of klebsiellae was especially marked.