Sequential samples of colostrum, transitional and mature milk from 47 women were investigated to determine the concentrations of 11S IgA, IgG, IgM, alpha 1-antitrypsin, lactoferrin, lysozyme, B1A globulin (C3) and B1E globulin (C4) by single radial immunodiffusion. These were all found to be high in colostrum with a decline in concentration as lactation proceeds although a wide variation in concentration was noted between different individuals at any given post-partum time interval. For each time interval considered, parity, age and maturity of pregnancy did not appear to influence significantly the variation in protein concentrations. Statistical analyses of the results obtained from samples taken during the first four days of lactation, when protein concentrations were highest, showed that parity and age did influence the variation in antimicrobial protein concentrations when examined using an analysis of variance but significantly higher concentrations of protective proteins were not consistently seen in the milk from any one or more parity or age groups. Mean concentrations of all proteins excluding IgA were found to be greater in those women delivering before their expected date of confinement during the first four days of lactation. This was statistically significant for C3 and C4 and approaching significance (p less than 0.08) for alpha 1-antitrypsin.