The primary purpose of the experiments reported in this paper was to determine whether a powerful systemic antigenic stimulus, given at the time of mammary involution, would affect the level of specific antibody in milk secreted during the ensuing lactation. The results showed that a single systemic administration of antigen with adjuvant, during mammary involution, is an efficient means of inducing high titres of specific antibody in milk in the subsequent lactation. Furthermore, such antibody belongs mainly to the IgA and IgM classes of immunoglobulin. These results suggest that the antibody is produced locally in the mammary gland by sensitised cells of the lymphocyte-plasma cell series, which infiltrate the tissue during the early stages of mammary involution and then become sessile.