The in vitro activity of FCE 22101, a new semisynthetic penem derivative, was compared with that of ceftriaxone, moxalactam, imipenem (formerly imipemide, N-formimidoyl thienamycin, or MK 0787), cefuroxime, ceftazidime, and other beta-lactams, when appropriate, against 472 recent isolates and known beta-lactam-resistant strains. The minimum inhibitory concentrations of FCE 22101 against 90% of the members of the family Enterobacteriaceae, Haemophilus influenzae, Staphylococcus aureus, Lancefield group D streptococci, and Bacteroides spp. were between 0.5 and 4 micrograms/ml. Methicillin-resistant strains of Staphylococcus aureus were susceptible. Ninety percent of the Neisseria gonorrhoeae and Streptococcus pneumoniae strains were susceptible to 0.25 microgram of FCE 22101 per ml. Pseudomonas aeruginosa strains were resistant to FCE 22101 (minimum inhibitory concentration, greater than 128 micrograms/ml). The susceptibility of known, characterized beta-lactamase-producing strains of the Enterobacteriaceae suggested that FCE 22101 is resistant to many beta-lactamases. Generally, FCE 22101 was slightly less active than imipenem, moxalactam, ceftriaxone, and ceftazidime against members of the Enterobacteriaceae and considerably more active than the cephalosporins (including moxalactam) against Staphylococcus aureus. The human serum protein binding of FCE 22101 was about 40%, and human serum had little effect on the activity.