Intravenous clindamycin phosphate therapy for severe anaerobic infections at doses of 300 to 450 mg every six to eight hours was evaluated in 42 patients, including 19 with bacteremia. Mean serum levels of clindamycin at peak and valley concentrations (15.2μg and 4.6μg/ml, respectively) well exceeded the median minimal inhibitory concentration (MIC) of clindamycin for all isolates tested (0.17μg/ml). Mortality in bacteremic patients treated with clindamycin was 21% and compared favorably with that of 27% in 48 patients with Bacteroidaceae bacteremia treated with chloramphenicol. Healing of infected sites in nonbacteremic patients occurred in 91% (21 of 23) and bacteriological cure was achieved in 97% (31 of 32) of all cases with follow-up cultures. Therapy was well tolerated without serious toxic reaction. These results indicate that parenterally administered clindamycin may replace chloramphenicol as the drug of choice for initial antimicrobial therapy of suspected anaerobic sepsis.