59 patients who had earlier developed an infection following antineoplastic chemotherapy were randomised to receive either granulocyte colony-stimulating factor (G-CSF) alone or G-CSF + quinolone as prophylaxis during subsequent identical chemotherapy courses. 30 patients received 48 courses of G+CSF, while 29 patients received 44 courses of G-CSF + ofloxacin or ciprofloxacin. The overall infection rate was 23%. Patients with WHO grade IV leukopenia at the onset of prophylactic treatment developed infection in 61% of cases when on G-CSF, but only in 22% when on G-CSF+quinolone (P = 0.002). Patients with initial leukopenia of grade WHO III-I had only a 11% infection rate showing no significant difference between the treatment groups. The median duration of leukopenia < 1 x 10(9)/l was 4 days for patients receiving G-CSF alone and 3.5 days for those receiving additional quinolone. Patients developing infection had grade IV leukopenia for a median of 5 days. Both prophylactic treatments were well tolerated. We conclude that when prophylactic G-CSF is initiated at WHO grade IV leukopenia, addition of an oral quinolone reduces the risk of infection.