Summary: Monocytes and polymorphonuclear neutrophils of human colostrum were identified using cytochemical staining procedures. The ability of colostral phagocytes to function in antibody dependent cellular cytotoxicity was found to be satisfactory at high effector/target cells ratio (20:1) Colostral monocytes showed a diminished cytotoxicity when compared to monocytes of adult or neonatal peripheral blood at low effector/target cells ratio (1:1). This could be due to the extensive fat ingestion by these cells or other inhibitory factors found in colostrum. Speculation: The demonstration of antibody dependent cellular cytotoxicity of colostral phagocytes gives further support to the biological importance of the cellular components of human milk.