Lymphocytes isolated from the peripheral blood and colostrum of 17 healthy donors, 1 to 3 days post partum, were cultured with cytomegalovirus (CMV), strain AD169, as stimulating antigen in a lymphocyte transformation test. The test was performed in microculture utilizing the cell-free supernatant of CMV-infected human fibroblasts and isolated peripheral blood and colostral lymphocytes showing normal reactivity to phytohemagglutinin. Lymphocytes from CMV-seropositive donors were stimulated by the CMV antigen, whereas lymphocytes from both male and female CMV-seronegative donors failed to respond, as measured by incorporation of tritiated thymidine. In those donors from whom utilizable colostrum samples were obtained, two donors lacking peripheral blood lymphocyte (PBL) CMV reactivity also lacked colostral lymphocyte CMV reactivity. The remaining five colostrum donors showed both PBL and colostral lymphocyte reactivity to the CMV antigen, with colostral lymphocyte reactivity in four of five donors exceeding PBL-cytomegalovirus reactivity. There was no clear-cut correlation between the titer of CMV antibody by complement fixation and the amount of tritiated thymidine incorporated into the lymphocytes stimulated with the CMV antigen. These results highlight current knowledge that milk and colostrum contain large numbers of specifically sensitized T-lymphocytes, expressing a selective quota of maternal cell-mediated immunities. We are currently investigating the possibility that CMV-reactive colostral lymphocytes obtained during suckling may be of some protective value to the immunologically inexperienced neonate by transferring an adoptive immunity. This assumes special significance in the light of known transmission of CMV via mothers' milk to neonates.