The effect of ferrous fumarate on the relative bioavailability of ciprofloxacin after a single 500 mg oral dose of ciprofloxacin was studied in eight healthy males. Blood samples were collected at regular intervals 0-24 h post-dose. Urine was collected during 24 h to determine the cumulative urine excretion of ciprofloxacin. Ciprofloxacin concentrations in serum and urine were determined by high pressure liquid chromatography. Mean area under the serum concentration-time curve decreased significantly (P less than 0.001) after ciprofloxacin was taken with 200 mg ferrous fumarate. The relative bioavailability was 30% when ciprofloxacin was given with ferrous fumarate. The maximum blood level decreased from 2.1 +/- 0.9 (control) to 0.6 +/- 0.2 mg/l (with ferrous fumarate). Further studies are needed to determine if chronic treatment with ferrous fumarate further decreases the relative bioavailability. For the moment administration of ciprofloxacin with ferrous fumarate should therefore be avoided.