The currently widely held theory that intrafollicular free fatty acids (FA) are the primary agents instigating inflammatory changes in acne is based on circumstantial evidence. There is no direct evidence that FA in physiologic concentrations are inflammatory. In the present study the quantities of FA present in isolated pilosebaceous ducts and in isolated comedones were analyzed. Using these values, the effect of FA on intracutaneous injection into human skin was investigated. The range of FA in 257 isolated pilosebaceous ducts from skin of the upper back of 10 male subjects was 0.19 to 2.43 mug, with an average of 0.89 +/- 0.75 mug of FA per duct. The mean FA content in 45 open comedones was 63.6 +/- 24.8 mug per comedone. Fatty acids for intracutaneous testing were isolated from human skin surface lipids and from hydrolyzed triglycerides purified from pooled isolated sebaceous glands. Twenty-six subjects received 100 mug of FA intracutaneously in the upper back. The response to FA injections could not be distinguished from the response to saline control injections. By 24 hr no erythema, induration, or any visible marks of inflammation were present in the skin of any of the subjects tested. At the histologic level a mild inflammatory infiltrate consisting perdominantly of lymphocytes was slightly more marked in the FA injection site than in the saline control injection site. Increasing the amount of FA injected to 500 mug still produced no visible inflammatory response in human skin. We conclude that intracutaneous injections of FA in physiologic concentrations do not produce more than a very mild inflammatory reaction in human skin and suggest that the role of Propionibacterium acnes in the pathogeneisis of acne may be more complex than merely as a source of intrafollicular lipases.