The in-vitro activity of cefpirome, cefotaxime, ticarcillin, piperacillin, and three aminoglycosides was compared by the broth dilution method against 248 consecutive clinical isolates of Pseudomonas aeruginosa and 35 aminoglycoside-resistant isolates of the Enterobacteriaceae. Cefpirome was more active against Ps. aeruginosa than cefotaxime and gentamicin, and as active as piperacillin, ticarcillin, tobramycin and amikacin. Ps. aeruginosa isolates resistant to a given aminoglycoside were significantly more resistant to cefpirome than isolates susceptible to that aminoglycoside. Other beta-lactam antibiotics were also less active against aminoglycoside-resistant strains of Ps. aeruginosa. Cefpirome was more potent than cefotaxime and the penicillins against aminoglycoside-resistant Enterobacteriaceae.