The activities of pirlimycin (U57930E), lincomycin, clindamycin, erythromycin, josamycin, oleandomycin, and spiramycin were compared against strains of the Bacteroides fragilis group. Pirlimycin was the most active; the 90% minimal inhibitory concentration was 1 microgram/ml, and the activity range was 0.125 to 4 micrograms/ml. This drug was fourfold more active than any of the other drugs, including clindamycin. The minimal bactericidal concentration (range, 0.5 to 16 micrograms/ml) shows that pirlimycin behaves as a bacteriostatic antibiotic.