Antimicrobial proteins in maternal and cord sera and sequential samples of human milk were studied in a group of 60 Chinese women to determine the degrees of passive immunity transferred from women of different nutritional status to their infants. Maternal malnutrition was characterized by low values for wt/ht2 and serum total protein and albumin. Maternal immunoglobulin (IgG) concentrations were not significantly different between well- and malnourished groups prepartum but were significantly different postpartum. Mean concentrations of cord IgG and lysozyme from well- and malnourished groups were not statistically different. During the first 7 d of lactation and most stages thereafter, mean concentrations of IgA; complements C3 and C4, and lysozyme in milk from the malnourished group were only half of those of the well-nourished group. Antimicrobial proteins transferred via milk to newborns may be influenced by the mother's nutritional status.