The expression of Class II MHC antigens in the rat mammary gland was assessed by immunoperoxidase using monoclonal antibodies against rat homologues of the murine I-A and I-E antigens. Our results showed that, while I-A and I-E were inconspicuous in undifferentiated mammary epithelium in virgin rats, during early pregnancy, the ductal epithelium becomes intensely positive for both antigens. A monoclonal antibody specific for macrophages showed that they were abundant in the stroma but did not correspond to the epithelial staining. During late pregnancy and lactation, the ductal epithelium expressed varying levels of Ia, while the alveolar epithelial cells were consistently Ia-negative. Ia-positive mononuclear cells of round or dendritic morphology present in the alveolar areas were enumerated as cells per high power field (HPF). Maximal cell counts for I-A and I-E were seen in late pregnancy and early lactation, with a significant decline in mid-lactation. Counts were higher in intraepithelial locations than in the subjacent connective tissue at all stages. The possible role of these stage-specific alterations in regulating local immune responses and transfer of immune components into milk is discussed.