has been reported previously from this laboratory (Woodruff and Symes, 1962) that the growth of an A-strain mouse mammary carcinoma transplanted to isogenic hosts is retarded if the tumour-inoculated animals are given a sublethal dose of whole body irradiation (400 r) followed by an intravenous injection of spleen cells from a mouse of a different strain (CBA) immunized against the tumour; a significant anti-tumour effect was however achieved only at the cost of producing severe and often fatal graft-versus-host disease in the treated animals. In the experiments cited, irradiation alone, and injection of spleen cells without prior irradiation, were ineffective. It seemed likely that in the combined treatment the effect of the irradiation was to delay rejection of the transplanted spleen cells, but there was no formal proof of this. It appeared desirable, therefore, to make further observations with the same type of tumour but under conditions in which immunological rejection of the transplanted spleen cells could be avoided without recourse to irradiation or the use of immunosuppressive drugs. This requirement can be met by maintaining the tumour in the F1 hybrid of tw
