Six patients with newly diagnosed haematological malignancy were treated with ciprofloxacin 500 mg orally, as chemoprophylaxis during neutropenia. The mean serum ciprofloxacin concentrations, measured 1, 2, 3 and 4 h after dosage were reduced for up to ten days after the start of chemotherapy. The mean maximum serum concentration and AUC 0-4 h were reduced from 3.7 (95% confidence limits, 2.3-5.1) mg/l and 10.7 (6.2-15.2) mg/l.h pre-chemotherapy to 2.0 (1.4-2.6) mg/l and 5.7 (4.4-7.0) mg/l.h, 13 days after chemotherapy. Absorption of ciprofloxacin is reduced after cytotoxic chemotherapy, and this may have implications for the use of oral ciprofloxacin in this group of patients.