Polymorphonuclear leukocytes (PMN) from human colostrum were compared with blood and oral exudate PMN from the same donors for their locomotive, respiratory burst, phagocytic, and shape change (polarization) capabilities. Blood PMN were functionally superior to PMN from other sites. Colostrum PMN were similar to oral exudate PMN in all areas except locomotive responses. Exposure of blood PMN to aqueous human colostrum resulted in decreased stimulated adherence to plastic, decreased bactericidal activity against Staphylococcus aureus, reversible induction of cellular shape change, and reversible decreases in cellular deformability. The colostrum effects on PMN shape change and deformability were accompanied by significant increases in cytoskeleton-associated actin. PMN isolated from colostrum have suppressed functions, consistent with their being exudate cells. In addition, the colostrum environment effectively suppresses multiple functions in PMN from blood, these effects being mediated in part by rapid cytoskeletal assembly. PMN in colostrum do not appear to be beneficial to the breast-fed infant due to deficiencies in function.