Cefsulodin was the most active of the cephalosporins and exhibited 4-16 times more activity than carbenicillin or ticarcillin against 50 clinical isolates of Pseudomonas aeruginosa. Azlocillin and piperacillin showed good activity, while tobramycin was the most effective aminoglycoside. The activity of cefsulodin was unaltered by increases in inocula, but resistance was induced easily. When combined with gentamicin, no synergistic or antagonistic activity was observed against multiply resistant isolates.