24 patients with severe infections were treated with intravenous minocycline 100 mg every 12 hours. Average blood levels were within therapeutic ranges during the first 12 hours after the initial dose. Determination of efficacy of therapy in 23 of the patients who were evaluable showed that clinical and bacteriological results were satisfactory in 20 patients, unsatisfactory in 2, and questionable in 1. One patient developed a fatal secondary infection which may have been related to prior therapy with minocycline. No toxicities or side-effects were observed.