Random allocation of 22 patients with benign and malignant diseases with neutrophil counts of up to 1 X 10(9)/l blood and probably infection-caused fever of more than 38 degrees C to intravenous treatment with one of the following antibiotic combinations was performed: carbenicillin (6 g/m(2) . 6 h) plus sisomicin (45 g/m2 . 6 h) or mezlocillin (3 g/m2 . 6 h) plus sisomicin (45 g/m2 . 6 h). Both combinations were tolerated equally well. Patients became afebrile in 16 out of 23 treatment periods. Seven out of 11 patients responded to carbenicillin - sisomicin, and 9 out of 12 to mezlocillin - sisomicin. Mezlocillin thus leads to equal success of treatment in febrile neutropenia as the double dose of carbenicillin when both antibiotics are combined with the same aminoglycoside.