Fc receptors on the surface of milk leukocytes from normal glands, bronchial leukocytes, mastocytoma P-815 cells, and murine leukemia L1210 cells were blocked significantly (P less than 0.01) by cavian and bovine milk collected from inflamed glands (mastitic milk), their wheys, and in vitro-prepared immune complexes composed of the whey from normal milk and serum. Blocking of Fc receptors indicated the presence of immune complexes in the mastitic milk and was detected by inhibition of rosette formation with sensitized erythrocytes or attachment of the aggregated immunoglobulin G. The binding of immune complexes to these cells was also determined by staining with fluorescein isothiocyanate-labeled protein A. As the mastitis subsided, the blocking effect of the mastitic milk also declined markedly. There was no significant difference in blocking capacity between mastitic milk and its whey. The blocking capacity of normal cavian or bovine milk and their wheys was insignificant. Whey from mastitic milk also inhibited phagocytosis of opsonized staphylococci by alveolar macrophages. We suggest that the blocking of Fc receptors on phagocytic cells adversely affects phagocytosis.