Colostrum and breast milk samples were obtained from 74 women, 18 of whom gave sequential samples. The mean total leukocyte count in colostrum was 3190 cells/mm3. Proportions of macrophages, polymorphs and lymphocytes varied widely; macrophages usually predominated. Serial sampling showed (1) a small fall in total counts through delivery, (2) a fall in total counts and the proportion of PMNs at the onset of lactation, (3) after 1 to 2 weeks of lactation the appearance of cytoplasmic fragments together with epithelial cells which later constituted the main cell type. It was estimated that the total number of leukocytes available to the neonate remained approximately constant during the first 2 weeks of lactation and fell thereafter. Functionally, morphologically and histochemically macrophages in colostrum and breast milk resembled macrophages elsewhere. Their ultrastructure was characterized by filiform surface projections, numerous endocytic vacuoles and lipid droplets in the cytoplasm.