The inhibitory activity of penem FCE 22101 was compared with those of imipenem and other relevant antibiotics against over 500 clinical isolates of Gram-positive cocci and anaerobes. The relative bactericidal activities of FCE 22101 and imipenem were compared by a killing curve method. FCE 22101 showed good inhibitory activity against most aerobic Gram-positive cocci although generally less than imipenem. A substantial number of Staphylococcus aureus isolates (43%) were highly susceptible to imipenem (MIC less than 0.0015 mg/l); the mode MIC for FCE 22101 was 0.06 mg/l. Methicillin-resistant Staph. aureus (MRSA) were often slightly more susceptible to FCE 22101 than to imipenem. Streptococci were more susceptible to imipenem than to FCE 22101; mode MICs for group A streptococci were 0.003 and 0.03 mg/l and for enterococci 1 and 4 mg/l, respectively. The anaerobic organisms tested were equally susceptible to both FCE 22101 and imipenem. Imipenem and FCE 22101 showed similar bactericidal activity at a concentration equivalent to 4 x MIC. Fully susceptible staphylococci were killed rapidly by both compounds, whereas less susceptible isolates, especially MRSA, were killed slowly. Streptococci, other than Str. pneumoniae, were also killed relatively slowly. Bacteroides fragilis group organisms were rapidly killed by both FCE 22101 and imipenem.