Patients were randomly assigned to receive carbenicillin plus tobramycin by continuous infusion (C+T), carbenicillin plus cefamandole by continuous infusion (C+CC) or carbenicillin plus cefamandole by intermittent infusion (C+IC) during 490 febrile episodes. Carbenicillin was administered over 2 hours every 4 hours. The per cent of cures achieved during the 235 documented infections was 65 per cent for C+CC, 57 per cent for C+IC and 54 per cent for C+T. Among those infections caused by single gram-negative bacilli, C+CC produced a higher cure rate than C+IC or C+T(74 per cent versus 59 per cent versus 50 per cent). C+CC was significantly more effective than C+IC among patients with persistent severe neutropenia of less than 100 neutrophils/mm3 (65 per cent versus 21 per cent, p = 0.03). If the infecting organism was sensitive to both antibiotics, the cure rate which occurred during 12 per cent to 13 per cent of the febrile episodes, regardless of antibiotic regimen. However, it occurred significantly more often during documented infections than during fevers of unknown etiology (20 per cent versus 6 per cent, p less than 0.001). C+CC appears to be the most effective of the three regimens for the treatment of infections in patients with persistent severe neutropenia.