Sparfloxacin (SPFX), a new oral quinolone antimicrobial, was studied for the bacterial response, pharmacokinetics and clinical efficacy in the treatment of bacterial prostatitis. The minimum inhibitory concentration (MIC) values for 48 strains isolated from expressed prostatic secretion were measured. The values for 8 out of 12 strains of S. epidermidis were below 0.05 microgram/ml and those for all 6 strains of E. coli were below 0.025 microgram/ml. The SPFX concentrations in prostatic fluid (PF) were 0.33 to 0.49 microgram/ml at 1 to 3 hours after oral administration of 200 mg, the PF/serum ratio being 1.15 to 1.47. SPFX was administered at a dose of 200 to 400 mg daily for an average of 14.1 days to 14 patients with prostatitis (5, acute: 9, chronic). The clinical efficacy judged by physician in charge was effective in 12 cases with an efficacy rate of 85.7%. The bacterial eradication rate was 93.3% (14/15 strains), and eradication was complete in all 7 cases infected with gram-negative rods. SPFX-related abnormal laboratory values were observed in one case with transient increase of glutamic-oxalacetic transaminase, glutamic-pyruvic transaminase and alkaliphosphatase activities, and decrease of platelet. As side effects, one case with gastrointestinal symptoms and the other case with photosensitivity skin rash accompanied by sensory abnormality of palms were observed. The abnormal values or side effects in these patients recovered to normal or disappeared after completion of the treatment without any treatments. In view of the higher concentrations in PF than the MIC values with long remaining in the tissues, SPFX is considered to be effective in the treatment of bacterial prostatitis.(ABSTRACT TRUNCATED AT 250 WORDS)