Susceptibilities of 104 Propionibacterium acnes isolates to each of 22 antimicrobial agents was evaluated by broth microdilution. These isolates were susceptible to all of the test agents except metronidazole. N-Formimidoyl thienamycin, a penem coded Sch 29482, and penicillin ranked first, second, and third, respectively, in activity and were significantly more active than other penicillins, cephalosporins, tetracyclines, clindamycin, or chloramphenicol.