An agar dilution method was used to measure the minimal inhibitory concentrations of 13 P-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 ofPseudomonas aeruginosa were inhibited by ceftazidime, N-formimidoyl thienamycin, and cefsulodin. N-Formimidoyl thienamycin was active against all of the species tested.A large number of 1-lactam antibiotics have become available during the last few years, and the in vitro activities of many of these drugs have been studied. Since in most of these studies only five or six antibiotics were evaluated together, we decided to compare the in vitro activities of 13 P-lactam antibiotics in one study. MATERIALS AND METHODSAntibiotics. Cephalothin and cefamandole were obtained from Lilly Research Centre Ltd., Windlesham, England; moxalactam was obtained from Eli Lilly & Co., Indianapolis, Ind.; cefuroxime and ceftazidime were obtained from Glaxo Group Research Ltd., Greenford, England; cefoxitin and N-formimidoyl thienamycin were obtained from Merck Sharp & Dohme Research Laboratories, Rahway, N.J.; ceftizoxime was obtained from Fujisawa Pharmaceutical Co. Ltd., Osaka, Japan; ceforanide was obtained from Bristol-Myers B.V., Bussum, The Netherlands; cefotaxime was obtained from Roussel Uclaf, Paris, France; cefoperazone was obtained from Pfizer Corp., Brussels, Belgium; cefsulodin was obtained from Ciba-Geigy Ltd., Basel, Switzerland; and ceftriaxone was obtained from Hoffmann-La Roche & Co. A.G., Basel, Switzerland.Bacterial isolates. The strains examined were cultured from patients hospitalized in the St. Radboud University Hospital, Nijmegen, The Netherlands, with the exception of most of the Campylobacter and Yersinia strains. The latter strains were human isolates obtained from the National Institute of Public Health, Bilthoven, The Netherlands. Most of the isolates were considered to be clinically significant, with the exception of the Citrobacter and Acinetobacter isolates, a number of the Streptococcus faecalis isolates, and the coagulase-negative Staphylococcus strains. The strains were collected during 1980. A number of strains were kept frozen at -70°C until they were used. Gonococci and Haemophilus influenzae were stored in 1% proteose peptone (catalog no. 0122; Difco Laboratories, Detroit, Mich.) containing 8% glycerin, and Campylobacterjejuni, Bacteroides spp., and Clostridium spp. were stored in fluid thioglycolate medium (catalog no. 0256; Difco). The remaining strains were stored at room temperature in the dark; streptococci were stored in cooked meat medium (catalog no. 0267; Difco), and the other strains were stored on slants of heart infusion agar (catalog no. 0044; Difco).Susceptibility tests. Minimal inhibitory concentrations (MICs) were determined by an agar dilution method. The susceptibility of C. jejuni was ...