The in vitro activity of thienamycin, a β-lactamase stable antibiotic against multiresistant Enterobacteriaceae, pseudomonas and Staphylococcus aureus has been compared with β-lactamase stable agents—cefotaxime, cefamandole and cefoxitin. Thienamycin was not more active than cefotaxime against facultative Gram-negative organisms, but was more active than cefamandole and cefoxitin against multiresistant β-lactamase bacteria and more active against Bacteroides fragilis than cefoxitin, cefotaxime and ticarcillin. Thienamycin also displayed potency against Staph aureus and enterococci that merit further evaluation when a stable congener is developed.