In a double-blind study on 20 patients with acne vulgaris, 10 patients were treated with topical clindamycin and 10 patients with oral tetracycline for eight weeks. The skin and colon microflora were determined before, during and up to eight weeks after treatment. All patients improved markedly to the same extent. Topical clindamycin caused no changes in the colon flora, whereas oral tetracycline significantly suppressed the numbers of colon bacteria and four patients were colonized by new tetracycline resistant bacteria. In patients receiving tetracycline, 40% of the skin bacteria became resistant during therapy, while the corresponding figure for clindamycin was 60%. The skin flora was normalized in most patients after the treatment had stopped.