Samples of precolostrum (colostrum gravidarum), colostrum and mature milk obtained from five women during their antenatal and postnatal periods were measured for IgA, IgG, IgM, alpha-1-antitrypsin, lactoferrin, lysozyme, B1A globulin (C3) and B1E globulin (C4) by single radial immunodiffusion. Protein concentrations in precolostrum were equal to or greater than those found in colostrum obtained during the first 12-48 hours following delivery. Secretion of precolostrum is common, occurs early in the antenatal period and may often be of considerable volume. The anti-microbial proteins contained within this milk can be preserved intact by freezing. This represents an untapped pool of bacteriostatic proteins with specific activity against neonatal pathogens. We suggest that a potential protective effect against serious infection may be obtained by administering precolostrum to "at risk" infants during the first few days of life.