Cells from human colostrum, collected from mothers within 48 hours of delivery, were examined for their capacity to phagocytose and kill Eschericia coli and Candida albicans. The phagocytic power of colostral cells was comparable to that of blood leukocytes from the same individuals. In contrast, the capacity of colostral cells to kill microorganisms was significantly less than that of blood leukocytes. Preincubation of blood leukocytes with colostrum supernatant did not reduce phagocytic indices, but reduced E. coli killing by 40% and C. albicans killing by 66%. The role of colostral cells in protecting the neonate from infection is discussed in the light of these findings.