The ability of horse red blood cell (HRBC)-specific T cells from mice expressing humoral immunity to suppress the induction of HRBC-specific delayed-type hypersensitivity (DTH) was investigated. The transfer of Ig-negative spleen cells, from mice injected 4 days previously with HRBC, completely suppressed the development of DTH in mice treated with cyclophosphamide and sensitized with HRBC. The suppressor cell was found to be lysed by treatment with anti-theta serum and complement. Furthermore, hemocyanin-specific immune T cells were able to suppress the DTH induced to HRBC, provided these two antigens were coupled together. These studies suggest that T cells present under conditions were humoral immunity is induced can suppress DTH and that such cells play an important role in the regulation of the immune response.