The in vitro activity of the new parenteral cephalosporin BMY-28142 was compared with that of cephalothin, cefoxitin, cefotaxime, ceftriaxone, moxalactam, aztreonam and ceftazidime, against a total of 374 recent multiresistant Gram-negative microorganisms of nosocomial origin. Against all species of Enterobacteriaceae resistant to the first- and second-generation cephalosporins, BMY-28142 had superior inhibitory activity than the newer beta-lactams tested, with intrinsic activity against E. coli and Pr. mirabilis slightly less than that of cefotaxime and ceftriaxone. BMY-28142 differed from the other beta-lactams mainly in being at least 16-fold more active against E. cloacae, while BMY-28142 and ceftazidime showed comparable activity against Ps. aeruginosa strains. Against strains of Ps. aeruginosa resistant to both BMY-28142 and amikacin, the combination of the two antibiotics proved to be synergistic in vitro.