In order to evaluate the possible damaging effect of pefloxacin on erythrocytes in subjects with G-6-PD deficiency, a trial on red-cell survival was carried out using a 51Cr-labelled peripheral red blood cell technique. Eight men, aged 52 to 64 years, with osteoarthrosis and G-6-PD deficiency (Mediterranean type) were treated with 800 mg/day pefloxacin. Pefloxacin did not modify red cell survival in the subjects under study. In all subjects erythrocyte GSH levels were investigated in baseline conditions and also after incubation with increasing doses of pefloxacin. No significant change in erythrocyte GSH levels was observed after incubation with therapeutic doses of pefloxacin.