Milk fat globules (MFG), which are formed by exocytosis of lipid from epithelial cells of the mammary gland, are enveloped by plasma membrane from the epithelial cells. Highly purified, detergent-solubilized MFG membranes have been shown to contain molecules reactive with a rabbit antiserum against HLA-DR antigens. Indirect immunoprecipitation combined with polyacrylamide gel electrophoresis in sodium dodecyl sulfate demonstrated that the MFG membrane material reactive with the antiserum comprised molecules which under denaturing conditions displayed molecular weights of 28,000 and 35,000. The two types of polypeptide chains, which were both glycosylated, were held together by noncovalent forces under nondenaturing conditions. Various types of chemical and physicochemical analyses failed to reveal any significant differences between the HLA-DR-like antigens from MFG and from spleen cells. Since the HLA-DR-like antigens bound detergent in micellar form and were expressed on the outside of intact MFG, as revealed by indirect immunofluorescence, it is concluded that these antigens are embedded in the hydrocarbon matrix of the MFG and not merely passively adsorbed onto the MFG.