The in vitro activities of moxalactam, GR 20263, and N-formimidoyl thienamycin were compared to those of cephalothin, cefazolin, cefamandole, cefoxitin, and tobramycin against 152 strains of Enterobacteriaceae and staphylococci. The results showed that moxalactam, GR 20263, and N-formimidoyl thienamycin each had significantly improved activity toward the gram-negative organisms tested compared to the other β-lactams and tobramycin. N-formimidoyl thienamycin was particularly impressive with respect to its activity toward Staphylococcus auteus and S. epidermidis as compared to moxalactam, GR 20263 and the older β-lactam drug