Class I MHC protein primarily presents endogenous antigen but also may present exogenous antigen. Here, we investigated the intracellular pathway of spontaneously internalized class I MHC protein by confocal microscopy. β₂-microglobulin (β₂m), labeled with a single fluorophore, was exchanged at the surface of B cell transfectants to specifically mark cell surface and endocytosed class I MHC protein. Intracellular β₂m colocalized with fluorophore-conjugated transferrin, implying that class I MHC protein endocytosed into early endosomes. These endosomes containing fluorescent β₂m were found close to or within the Golgi apparatus, marked by fluorescent ceramide. Even after 24 hr of incubation, very little fluorescent β₂m was found in intracellular organelles stained by DiOC₆, marking the endoplasmic reticulum, or fluorophore-conjugated low density lipoprotein, marking late endosomes and lysosomes. Fluorophore-conjugated superantigens (staphylococcal enterotoxin A and B), presumed to enter cells bound to class II MHC protein, also were found to endocytose into β₂m-containing early endosomes. Staining with mAb and use of transfectants expressing MHC protein attached to green fluorescent protein confirmed the presence of intracellular compartments rich in both class I and II MHC protein and demonstrated that class I and II MHC protein also colocalize in discrete microdomains at the cell surface. These cell surface microdomains also contained transferrin receptor and often were juxtaposed to cholesterol-rich lipid rafts. Thus, class I and II MHC protein meet in microdomains of the plasma membrane and endocytose into early endosomes, where both may acquire and present exogenous antigen.