A minor population of alpha beta T cells expresses neither CD4 nor CD8. These cells are probably heterogeneous. They are widespread in anatomical distribution, express an abnormal T-cell repertoire largely unaffected by selective processes that act on CD4+ and CD8+ T cells, and exhibit both cytotoxic and Th-2-like functions. Their role in normal immune function is completely obscure, but they are active in murine and human autoimmune disease.