To study the mechanism of action of erythromycin stearate in acne vulgaris, the composition of the skin surface lipids was analyzed before, during and after treatment (500 mg daily for 1 week, 250 mg daily for 2 months) in 17 patients. Quantitative thin-layer chromatography showed that the fatty acid fraction in the skin surface lipids decreased significantly during the treatment period. The clinical effect was good in 14 of 17 patients. Further investigations were performed on the effect of erythromycin stearate on purified pancreatic lipase. The enzyme activity was not inhibited by exposure to the drug.