Clindamycin was used to treat six patients with endocarditis because of allergy to penicillin in five, and an unfavorable clinical response to methicillin in one. Only one patient had an uneventful cure with clindamycin. Two had hepatotoxicity which resolved rapidly after clindamycin was stopped. Two patients, one of whom had an aortic prosthesis, had completed four to six weeks of clindamycin therapy when clinical relapse occurred and blood cultures were again positive for a clindamycin-sensitive isolate. A fifth patient had peptostreptococcal endocarditis. Despite a favorable initial clinical and bacteriologic response, blood cultures taken on the 20th day of therapy again grew the Peptostreptococcus. This relapse pathogen had become resistant to clindamycin and was 100-fold less sensitive than the initial isolate. The few conditions in which clindamycin is indicated for therapy of bacterial endocarditis are outlined.