Professional antigen-presenting cells (APCs) have a distinct compartment in which class II molecules are proposed to acquire antigenic peptides. Genetic evidence suggests that human leukocyte antigen (HLA)-DM, an unusual class II molecule, participates in this process. Peptide acquisition was reconstituted in nonprofessional APCs by transfection of class II, invariant chain (Ii), and H-2M, the murine equivalent of DM. The H-2M heterodimer appeared in an endosomal compartment, not at the cell surface, and the localization was independent of Ii. The data presented show that H-2M, class II, and Ii are the minimally required components for efficient formation of stable class II-peptide complexes, and thus for a functional class II compartment.