Clindamycin, tinidazole (a parent compound to metronidazole), and doxycycline were compared in vitro against 376 anaerobic bacteria isolated from clinical specimens. Bacteriostatic tests indicated that clindamycin was the most active drug, on a weight basis, against these strains except for Clostridium species. The three drugs were compared as therapies for anaerobic wound infections in cancer patients. In a randomized double-blind study, no statistically significant differences between clindamycin and tinidazole could be documented. Doxycycline was less active presumably because of the lack of clinical response in three out of four patients infected with doxycycline-resistant strains. No major untoward effects were observed. The bactericidal dilution of the serum was predictive of the clinical outcome.