The subpopulations of lymphocytes in the pregnant and non-pregnant mammary glands of the sheep were delineated by a panel of monoclonal antibodies. The most striking feature observed was that in the mammary gland of both pregnant and non-pregnant sheep the great majority of the lymphocytes in the ductal and alveolar epithelium were agranulated CD8+ CD5- cells. A small subpopulation of granulated lymphocytes in the epithelium expressed the CD45R antigen but not the major histocompatibility complex (MHC) class II molecules. Other subpopulations, especially B lymphocytes, were present in much lower concentrations and were located mainly in the periductal and intralobular connective tissues. Patches of lymphocytes clustering around venules were observed and the majority of them were shown to be CD5+ CD4+, while some were CD5+ CD8+ but none were CD45R+ (B cell). It is suggested that selective traffic of T cells occurs at these sites.