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In the past few years there has been a resurgence of interest in the protein composition of human milk. Up to now the influence of maternal diet and of the mothers' nutritional status on the protein composition of human milk have not been fully clarified. We have evaluated the relationship between the mothers' socioeconomic status and weight and the protein composition of human milk. Protein fractions were determined by a polyacrylamide gel electrophoresis method in 181 samples of human milk obtained from voluntary donors. Samples were classified according to the time of lactation and in relation to the socioeconomic status and to the weight of the lactating women. Total protein and non-protein nitrogen decreased with advancing lactation but there were no differences among the socioeconomic and weight groups of mothers who were considered. beta- and kappa-caseins fell during lactation and beta-casein was significantly increased in the milk of the upper socioeconomic class with respect to that of the low one. alpha-lactalbumin increased from transitional to mature milk (16-30 d) and then declined. The milk from the low socioeconomic group presented the lowest levels of this protein. Lysozyme increased during lactation, whereas lactoferrin decreased. Both proteins were significantly influenced by the mothers' socioeconomic status; the highest concentrations for these proteins were found in the milk of the low socioeconomic group. Deficit or excess of mothers' weight did not influence the levels of the different protein fractions of human milk.