Recent studies in our laboratory have demonstrated that the great majority of human colostral T cells display the phenotypic and functional characteristics of memory T lymphocytes, e.g. were able to proliferate in response to anti-CD3 and anti-CD2 monoclonal antibodies, and to a lesser extent, to the lectin mitogen phytohaemagglutinin. In addition, their production of interferon-gamma after anti-CD3 and anti-CD2 stimuli was similar to that calculated in autologous blood lymphocyte cultures. More interestingly, the proportion of T lymphocytes bearing the gamma/delta T-cell receptor was found to be significantly higher in the mammary secretion than in autologous and heterologous blood samples. Furthermore, these cells were mostly delta-TCS-1+, thereby suggesting that they are actively motile cells capable of migrating from lymphoid to extra-lymphoid body tissues. The fact that the phenotypic pattern of colostral gamma/delta T cells is similar, if not identical, to that of the intestinal intraepithelial counterpart suggests that these cells might originate in the gut-associated lymphoid system and home selectively to the mammary gland late in pregnancy and throughout lactation. However, additional studies are needed to confirm whether milk T lymphocytes are actively involved in the adoptive lactation transmission of cellular immunity to the suckling infant.