Human milk lymphocytes (ML) can be partially purified and propagated in vitro as a means of assessing their immunological function. When exposed to a variety of stimuli known to activate T lymphocytes, ML respond in a unique manner that indicates a selected population of immunocompetent cells. ML are hyporesponsive to to nonspecific mitogens and respond in a reduced manner to histocompatibility antigens on allogeneic cells. In most cases, they are completely unresponsive to C. albicans although blood lymphocytes from the same patients respond to the antigen. The Kl capsular antigen of E. coli induces significant proliferation in lymphocytes obtained from milk, but fails to stimulate blood lymphocytes. This dichotomy of reactivity does not appear to result from suppressive factors or cells in milk or insufficient adherent cell function. Rather it appears to reflect the accumulation of particular lymphocyte clones in the breast and the local nature of mammary tissue immunity at the T-lymphocyte level.