Delayed-type hypersensitivity (DTH) responses to alloantigens were found to correlate with both skin and tumour allograft rejection in 224 reconstituted ATXBM-CBA mice. Furthermore, DTH responses and allograft rejection were observed only in mice that had received Ly-1 cells. Depletion of Thy-1+ or Ly-1+ cells led to indefinite graft survival and the absence of DTH responses, whereas depletion of Ly-2+ cells led to rapid graft rejection and strong DTH responses. The same result was obtained with CBA mice responding to grafts of either C57BL/6 skin, the B16 melanoma, or the EL4 lymphoma; and for (CBA XA) F1 mice responding to H-2K region alloantigens of AQR skin grafts. Thus, DTH and allograft rejection are both mediated by a Ly-1 T cell and it is considered that these are two different manifestations of the same transplantation response.