Wistar rats were infected by injection of 0.05 ml of a dense oily suspension of Staphylococcus aureus into the posterior thigh muscles of the hind leg. Three days later, solid abscesses had formed which were characterized by a peripheral accumulation of polymorphocytes and incipient central necrosis. At this time, 10 mg/kg of [14C]-ciprofloxacin (1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7-piperazin-1-ylquino line-3-carboxylic acid, Bay o 9867; designated tradename: Ciprobay) were administered intravenously. The animals were sacrificed at various time intervals after treatment and the distribution of radioactivity was examined by whole-body autoradiography. Five min after administration of ciprofloxacin, the radioactivity was found to be differentially distributed among all organs and tissues, but no radioactivity was detectable in the abscess. Beginning from 1 h post appl., increasing relative amounts of radioactivity were seen inside the abscesses. The relative enrichment as compared to the surrounding muscle tissue was most pronounced after 5 h, indicating that the radioactivity was eliminated more rapidly from the muscle than from the abscess. Some radioactivity was still present in the abscess 8 h after treatment of the animals. The comparison of autoradiograms and corresponding histological sections revealed a distinct affinity of [14C]-ciprofloxacin and/or its potential radioactive metabolites to the areas of inflammatory cellular infiltrates.