An agar dilution method was used to measure the minimal inhibitory concentrations of 13 beta-lactam antibiotics against 868 recent human clinical isolates. Most members of the Enterobacteriaceae were susceptible to cefoperazone, ceftazidime, moxalactam, N-formimidoyl thienamycin, ceftriaxone, and ceftizoxime. Cephalothin was the most active antibiotic against Staphylococcus aureus. Most strains of Pseudomonas aeruginosa were inhibited by ceftazidime, N-formimidoyl thienamycin, and cefsulodin. N-Formimidoyl thienamycin was active against all of the species tested.