Twenty multiple resistant clinical isolates were tested with N-formimidoyl thienamycin, moxalactam, cefotaxime, cefoperazone, and the three ureidopenicillins: azlocillin, mezlocillin, and piperacillin. A concentration of < 0.97 Itg/ml inhibited 100% of organisms for N:f-thienamycin and cefotaxime, 90% for moxalactam, and 60% for cefoperazone. An increase in inoculum from 10 3 to 10 6 cells reduced activity fourfold for 95% of isolates with cefoperazone, 70% with N-formimidoyl thienamycin, 65% for cefotaxime, but only 15% for moxalactam. For ureidopenicillins, 85% of strains tested had MIC's-c;, 15.6 Itg/m!. An inoculum effect was observed in only 35-50%. At 10 3 , the cidal concentration was the same or twofold greater than the inhibitory level for N:f-thienamycin and cephalosporins in 70% of strains tested and 65% for penicillins. With 10 6 , the 70% value remained for N:f-thienamycin but was reduced to 45% for cefotaxime and 25% for moxalactam; 85% demonstrated>eightfold differences with cefoperazone. Single step high-level resistance was observed to moxalactam (20%). Carbenicillin resistant strains were cross-resistant to the ureidopenicillins. N-fthienamycin and cefotaxime appeared comparable, although important differences between morphological alteration and metabolism may influence their therapeutic effectiveness.