Blood and mammary secretions were collected from involuted mammary glands of five primiparous Holstein cows to determine the effects of serum and mammary secretion skims on mononuclear cell proliferation. Proliferation was stimulated using concana‐valin A, phytohemagglutinin, pokeweed mitogen, and a mixed leukocyte assay. Autologous mammary secretion skims suppressed blood mononuclear cell proliferation. Suppression was reduced slightly when secretion was added 24 h following initiation of cultures. Addition of autologous serum enhanced mammary gland mononuclear cell proliferation markedly, in some cases equalling or exceeding responses of blood mononuclear cells from the same donors. Ability of serum to restore mammary mononuclear cell proliferation suggests that mammary mononuclear cells may be as competent as blood mononuclear cells if inhibitory effects of mammary secretion can be overcome.