843 isolates from clinical specimens were tested against moxalactam by disc agar diffusion. The bacteria used in this study consisted of Escherichia coli, Enterobacter aerogenes, Enterobacter agglomerans, Enterobacter cloacae, Klebsiella pneumoniae, Proteus mirabilis, Providencia rettgeri, Pseudomonas aeruginosa, Serratia marcescens, Staphylococcus aureus, Staphylococcus epidermidis and group B and group D Streptococci. In vitro activity of moxalactam was compared with the following antibiotics: ampicillin, amikacin, carbenicillin, cefamandole, cefoxitin, cephalothin, chloramphenicol, clindamycin, colistin, erythromycin, gentamicin, oxacillin, penicillin, tetracycline, tobramycin, trimethoprim-sulfamethoxazole and vancomycin. Of the 471 strains of Enterobacteriaceae tested, 466 (98.9%) were susceptible to moxalactam. Except for penicillin G, the gram-positive cocci were generally more resistant to moxalactam than the other beta-lactam antibiotics. Moxalactam was comparable to gentamicin, as far as its activity against P. aeruginosa was concerned, but was less effective than amikacin, tobramycin, carbenicillin or colistin.