Ten patients with newly diagnosed haematological malignancy were given ofloxacin, 400 mg daily orally, for antibacterial prophylaxis during neutropenia. Before cyto-toxic 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. © 1993 The British Society for Antimicrobial Chemotherapy.