The stimulation of naive donor T cells by recipient alloantigen is central to the pathogenesis of graft-versus-host disease after bone marrow transplantation (BMT). Using mouse models of transplantation, we have observed that donor cells become “cross-dressed” in very high levels of recipient hematopoietic cell–derived MHC class I and II molecules following BMT. Recipient-type MHC is transiently present on donor dendritic cells (DCs) after BMT in the setting of myeloablative conditioning but is persistent after nonmyeloablative conditioning, in which recipient hematopoietic cells remain in high numbers. Despite the high level of recipient-derived alloantigen present on the surface of donor DCs, donor T cell proliferative responses are generated only in response to processed recipient alloantigen presented via the indirect pathway and not in response to cross-dressed MHC. Assays in which exogenous peptide is added to cross-dressed MHC in the presence of naive TCR transgenic T cells specific to the MHC class II–peptide combination confirm that cross-dressed APC cannot induce T cell proliferation in isolation. Despite failure to induce T cell proliferation, cross-dressing by donor DCs contributes to generation of the immunological synapse between DCs and CD4 T cells, and this is required for maximal responses induced by classical indirectly presented alloantigen. We conclude that the process of cross-dressing by donor DCs serves as an efficient alternative pathway for the acquisition of recipient alloantigen and that once acquired, this cross-dressed MHC can assist in immune synapse formation prior to the induction of full T cell proliferative responses by concurrent indirect Ag presentation.