We performed basic and clinical studies on the effects of a new oral quinolone derivative, levofloxacin (LVFX, Code No. DR 3355) which is an optical l-isomer of ofloxacin, in acute epididymitis. LVFX was administered in a dose of 200 mg to prostatic cancer patients 2 hours before operation. The mean concentration of LVFX in the tissues of testis and epididymis were 4.73 micrograms/g and 313-3.6 micrograms/g, respectively. Tissue/Serum ratios were 1.63 and 1.16-1.32, respectively. LVFX was administered in a dose of 100 mg three times daily for 13 days to healthy male volunteers. Semen and blood samples were taken 2 hrs after 7th and last day of administration. The concentration of LVFX in semen were 1.19 micrograms/ml (7th day) and 1.32 micrograms/ml (13th day). Semen/serum ratios were 1.12 and 1.26, respectively. No affection of LVFX on the sperm was observed. Antimicrobial activity of LVFX to C. trachomatis showed good MICs of 0.25-1.0 micrograms/ml. LVFX was administered in a dose of 100 mg two or three times daily for 14 days to 23 patients with acute epididymitis. The overall efficacy rate based on a criteria for acute epididymitis showed 100% (excellent: 16, good: 4, 20/20). A better efficacy rate was obtained on the 14th day than 7th day. No subjective or objective adverse reactions were observed.