The components in human sebum that mediate the formation of comedones in the rabbit ear were evaluated by applying various sebum fractions as well as individual authentic components of sebum. Practically all the major constituents of sebum had some activity. The activities of the triglycerides and waxes were respectively moderate and weak. Among the saturated fatty acids activity began at C10, reached a peak at C12 and C14, lessened at C16 and C18, and was absent at C20. Monounsaturation increased comedogenicity but high unsaturation had the opposite effect. Irritancy and comedogenicity were shown to be unrelated properties. The lower fatty acids C3 and C5 were highly irritating and noncomedogenic while C16 and C18 were comedogenic and non-irritating. The sebum of various animals had borderline comedogenicity because of the absence of triglycerides and fatty acids.