Breast milk samples obtained from 74 women, at different stages of lactation, were analysed for leukocyte concentration and their bactericidal activity. The total leukocyte count in colostrum was 5 000/mm3 a concentration which is similar to that in circulation. As lactation became established, there was a drop in leukocyte concentration and the average count in mature milk was around 2 000/mm3 after the first 3 months. Polymorphs and mononuclear leukocytes together constituted 90--95% of the cells and the rest were lymphocytes. Bactericidal activity of milk leukocytes was similar to that of circulating leukocytes, irrespective of the stage of lactation. These results indicate that breast milk is rich in living leukocytes and that these cells may have a protective role against infection in the infant. The total leukocyte concentration as well as their bactericidal capacity were similar in well nourished and undernourished women suggesting that the protective factors in milk are not influenced by nutritional status of the mother.