Cefoperazone, a new broad-spectrum cephalosporin, was administered by continuous infusion or intermittent injection to 20 and 25 patients respectively with gram-negative bacillary septicemia most of whom had severe underlying disease. No difference was observed in the clinical response of the two groups. The overall rate of favourable response was 76%. Of the 40 non-neutropenic patients treated three died of infection despite sensitivity of the organism, three improved temporarily and three died as the result of superinfection. Three of the five neutropenic patients treated failed to respond to cefoperazone therapy, despite adequate serum bactericidal activity. Tolerance to cefoperazone was satisfactory. Emergence of cefoperazone-resistant strains leading to relapse or superinfection was not a major cause of failure.