A double-blind evaluation of the efficacy and safety of minocycline hydrochloride and tetracycline hydrochloride was conducted and completed using 49 patients with Pillsbury grade 2 or grade 3 acne. For six months, half of the patients received minocycline and half received tetracycline. Although the differences between treatment groups were not statistically significant at any evaluation, more patients treated with minocycline reached and maintained a noninflammatory acne status in less time than did patients treated with tetracycline. After six weeks, twice as many patients in the group treated with minocycline had reached noninflammatory status. Side effects reported by 7 patients were equally distributed between treatment groups. No notable abnormalities were observed in the results of blood chemistry studies, hematologic tests, quantitative serum immunoglobulin determinations, or thyroid function tests in 20 of the patients examined.