Ten patients with newly diagnosed haematological malignancy were given ofloxacin, 400 mg daily orally, for antibacterial prophylaxis during neutropenia. Before cytotoxic chemotherapy was given, and on three occasions between days 2-3, 5-7 and 8-10 afterwards, blood samples were taken before and 1, 2, 3, 4, 6 and 10 h after administration of ofloxacin. Serum ofloxacin concentrations were measured by HPLC. Mean (range) pharmacokinetic parameters before chemotherapy were: Cmax, 6.1 (2.7-9.1) mg/L; AUC0-2, 7.5 (1.9-11.6) mg.h/L. On days 2-3 post-chemotherapy the values were reduced to: Cmax, 5.0 (3.6-7.3) mg/L; AUC0-2, 5.8 (1.9-7.8) mg.h/L. On days 5-7 the values were: Cmax, 6.1 (4.0-10.4) mg/L; AUC0-2, 8.5 (0.8-16.8) mg.h/L, and by days 8-10 the values were: Cmax, 6.4 (3.3-12.5) mg/L; AUC0-2, 8.6 (0.8-23.1) mg.h/L. There was a statistically significant difference between Cmax pre- and Cmax two to three days post-chemotherapy (paired t-test, P = 0.05). These changes are similar to those previously reported with ciprofloxacin but less marked.