The relative safety and efficacy of minocycline and cephalexin were examined in patients with acute or chronic prostatitis. The multicenter study was of single-blind, parallel-group design. Forty-two men received minocycline (200-mg initial dose followed by 100 mg twice daily) and 44, cephalexin (500 mg four times daily); each antibiotic was administered orally for four weeks. A follow-up period of patient assessment extended for an additional six weeks. Evaluable data were available for 20 minocycline-treated patients and for 24 cephalexin-treated patients. Clinical cure or improvement without recurrence was seen in 65 per cent of the patients who received minocycline and in 46 per cent of those given cephalexin. Bacteriologic cure without relapse or reinfection occurred in 45 per cent of the minocycline-treated men and in 21 per cent of the cephalexin-treated men. Serious adverse clinical experiences were not encountered in either treatment group. Although several factors, mainly the small number of patients, precluded a statistical analysis of comparative efficacy, it was evident that more patients in the minocycline-treated group had both clinical and bacteriologic cures (35%) than did those in the cephalexin-treated group (21%).