Using the technique of homologous recombination in embryonic stem cells, a mouse strain without functional CD4 and CD8 genes has been generated. Surprisingly, these mice contain significant numbers of alpha beta T cells. Although mice deficient for CD8 only do not show any cytotoxic response when their T cells are stimulated with either alloantigen or viral antigen, the CD4-8- mice do generate alloreactive cytotoxic T cells. These cytotoxic T cells bear the alpha beta T cell receptor and recognize major histocompatibility complex class I antigens. In addition fully allogeneic skin transplants were rejected but skin transplants expressing only minor transplantation antigens were not. Virus-specific cytotoxic T lymphocytes were also not detected. It seems that alloreactive cytotoxic T cells can be induced and exert their effector function in vitro and in vivo in the absence of CD8, and that they can develop and mature in vivo without the CD8 molecule or the signals it might provide.