The results of minocycline and doxycycline therapy in 41 patients with chronic prostatitis and minocycline therapy in 6 patients with acute prostatitis were evaluated. In the comparative study of chronic prostatitis, minocycline and doxycycline were given on the same dosage schedule, milligram for milligram: a loading dose of 200 mg. followed by 100 mg. twicd daily. Over-all clinical responses to therapy with either agent were generally satisfactory, and no statistically significant difference was demonstrable in this regard. In the group with chronic prostatitis treated wtih minocycline, however, all symptoms manifested before therapy had disappeared after therapy even where over-all results had been judged unsatisfactory. This was not true of the group with chronic prostatitis treated with doxycycline. Symptoms in 6 patients persisted after therapy had been terminated. Here the difference in results between the two groups was found to be statistically significant. A review of symptoms two to eight weeks after therapy revealed no significant difference between the two groups. After two years only 6 patients in the entire group with chronic prostatitis had returned with recurrent problems: 3 of these patients had been treated with minocycline and 3 had been treated with doxycycline. Results of therapy in the small series of patients with acute prostatitis treated with minocycline were generally satisfactory.