Two allospecific uncloned T cell lines, C.C3.11.75 (H-2d anti-H-2k) and B6-C.7.76 (H-2b anti-H-2d), were assayed for their ability to elicit a DTH reaction in vivo. It was shown that both cell lines, C.C3.11.75 being cytolytic and B6-C.7.76 being noncytolytic, evoke a strong DTH reaction. The reaction was in tempo and histology typical for a DTH reaction and could not be distinguished from a DTH reaction elicited by in vivo generated T cells active in a GVH reaction. Both cell lines recognize a private specificity encoded in the H-2 IA subregion. Cell proliferation of the responder cells did not appear to be critical in the reaction nor are T cells from the host recruited during the reaction. The DTH reaction can be assayed in allogeneic hosts demonstrating a complete lack of H-2 restriction in the reaction even when presumably antigen presenting macrophages are removed from the stimulator cell population. Furthermore, tumor cells expressing IA antigens could be employed to cause the reaction in both syngeneic and allogeneic recipients. Cloned sublines of one of the lines are also active in the reaction.