The antibacterial in vitro activity of carumonam, a new monobactam, and cefpirome, a new cephalosporin, was studied on 483 hospital strains resistant to gentamicin and cephalothin, in comparison with amikacin, azlocillin, aztreonam, cefmenoxim, cefoperazone, cefotaxim, cefsulodin (for Pseudomonas), ceftazidime, ceftriaxone, cefuroxim, chloramphenicol, ciprofloxacin, doxycycline, enoxacin, netilmicin, norfloxacin, pefloxacin, piperacillin, rifampicin, tobramycin and trimethoprim. In general the two compounds have a very good in vito activity on Enterobacteriaceae but are less active on non-fermenting microorganisms. For the Enterobacteriaceae the minimal inhibitory concentrations 90% for carumonam was less than or equal to 1.1 mg/l excepted for Enterobacter spp. (43,6 mg/l) and M. morganii (56.8 mg/l) . All the Enterobacteriaceae are susceptible to cefpirome (minimal inhibitory concentrations 90% less than or equal to 5.3 mg/l). The activity of carumonam and cefpirome on Enterobacteriaceae is comparable with that of the third generation cephalosporins. Carumonam is more active than cefpirome and other beta-lactams, ceftazidime excepted, on Pseudomonas aeruginosa and Pseudomonas spp. On the other hand, both compounds reveal to have only a low activity on the other non-fermenters which minimal inhibitory concentrations 90% values of 115.4 mg/l for carumonam and 32.0 mg/l for cefpirome