SUMMARY The presence of H-2 antigenic activity has been detected in mouse brain by the ability of brain transplants to induce the accelerated rejection of skin allografts in congenic lines. Brain transplants are at least as antigenic as skin allografts, more antigenic than ovary allografts, but less antigenic than spleen allografts. These results suggest that in vivo methods may detect antigens that are not detected by in vitro methods