Lymphocytes were isolated from human colostrum and peripheral blood collected within 7 days postpartum. A 20-fold enrichment of lymphocytes from colostrum was achieved by centrifugal elutriation followed by equilibrium density gradient centrifugation. The distribution of T lymphocyte subsets was determined by immunofluorescence analysis of reactivity with monoclonal antibodies of the OKT series. In confirmation of previous reports employing sheep erythrocyte rosette assays, the majority of colostral lymphocytes reacted with OKT3 monoclonal antibody, a reagent that detects a surface antigen on mature T cells. Neither colostral nor blood lymphocytes from postpartum donors expressed the OKT6 antigen that is found in immature thymocytes. A novel finding is that the T lymphocyte population in colostrum contains both OKT4 (helper/inducer phenotype) and OKT8- (cytotoxic/suppressor phenotype) positive subsets. The relative ratio of OKT4 to OKT8-positive T cell subsets in colostrum, however, is generally lower than that observed for peripheral blood T cells.