The MHC class II gene Aa was disrupted by targeted mutation in embryonic stem (ES) cells derived from C57BL/6 mice to prevent expression of MHC class II molecules. Contrary to previous reports, the effect of the null-mutation on T cell development was investigated in C57BL/6 mice, which provide a defined genetic background. The complete lack of cell surface expression of MHC class II molecules in B6-Aa⁰/Aa⁰ homozygous mutant mice was directly demonstrated by cytofiuorometric analysis using anti-A^b and anti-la specific mAbs. Development of CD4+CD8− T cells in the thymus was largely absent except for a small population of thymocytes expressing high levels of CD4 together with low amounts of CD8. The majority of these cells express the TCR at high density. Although mature CD4+CD8− T cells were undetectable in the thymus, some T cells with a CD4+CD8−TCR^high phenotype were found in lymph nodes and spleen. Peripheral T cells from themutant mice can be polyclonally activated in vitro with the mitogen concanavalin A. However, they could not be stimulated with staphylococcal enterotoxin B in autologous lymphocyte reactions, thereby demonstrating the absence of MHC class II expression in these mice. Peripheral B cells in B6-Aa⁰/Aa⁰ mutants were functional and responded to the T cell independent antigen levan by the production of antigenspecific IgM antibodies similar to wild-type cells. The B6-Aa⁰/Aa⁰ mutant mice described in this study represent an important tool to investigate the involvement of MHC class II molecules in lymphocyte maturation and the immune response.