To study in mice the effects of in vivo xenogenic immunization with human major histocompatibility complex (MHC) class II antigens, the animals were injected with HLA-DR antigens and their proliferative responses tested in vitro. The results showed that small amounts of HLA-DR proteins, acting as nominal antigens, were not only able to prime mice for a secondary in vitro xenogenic mixed lymphocyte reaction but also induced a syngeneic mixed lymphocyte reaction. In contrast, allogeneic or syngeneic immunization of mice with soluble MHC class II molecules failed to stimulate an autoreactive response. The syngeneic mixed lymphocyte reaction was primarily directed against syngeneic MHC class II molecules since the murine T lymphocytes reacted against MHC class II-positive dendritic spleen cells and MHC class II-transfected mouse fibroblasts. A self-reactive T-cell line induced under these experimental conditions did not react in xenogeneic and allogeneic mixed lymphocyte reactions. However, these T lymphocytes proliferated when human peripheral blood lymphocytes of various haplotypes were presented in the context of syngeneic mouse antigen presenting cells.