Bacteria were sampled using a "scrub" technique from the skin surface of the faces of forty-nine female subjects aged 18-21 years. The sebum excretion rate was determined by a gravimetric method and the level of free fatty acids by titration. The production rate of free fatty acids was calculated from the product of the concentration of free fatty acids in the sebum and the sebum excretion rate. The date was analysed using Kendall's rank correlation method. Positive correlations existed between the number of Micrococcaceae and the skin propionibacteria (P < 0.001) and between both groups of organisms and the production rate of free fatty acids (P < 0.001). There was no significant correlation between the size of the bacterial population and the sebum excretion rate. The results support the view that free fatty acids are produced as a result of bacterial action, that the Micrococcaceae and skin propionibacteria do not compete to the detriment of their respective populations, and that the size of the bacterial population is not dependent upon the sebum excretion rate.