In order to test for further homologies between the MHC of mammals and amphibians, experiments were conducted to assess whether lower vertebrates such as anurans were able to generate killer cells after allogeneic stimulation. The generation of cytotoxic effector cells could be obtained in outbred families and clones of isogenic frogs after in vivo priming with either irradiated allogeneic lymphocytes or with an allogeneic skin graft, provided that the immune spleen cells were restimulated in vitro with the specific irradiated cells used for priming. Effector cells generated against a defined MHC haplotype could lyse targets having one of their two haplotypes in common with the stimulators. In contrast, no lysis was observed when the target cells differed from the specific stimulators by two MHC haplotypes. The cytotoxic activity of the MLR-restimulated lymphocytes appeared to be mediated by T cells since passage of the effector spleen cells through a nylon wool column, under conditions which removed Xenopus B lymphocytes, improved killing on a per cell basis. It therefore appears that the genes responsible for the highly specialized function of T killer cells have emerged early in evolution at least at the time of the emergence of the amphibians (≈300 million years ago) and that they were already linked to the MHC of this species. The MHC polymorphism in Xenopus seems to be lower than in mammals as evidenced by the high frequency of cross-killing observations paralleled by the high frequency of MLR identical animals found in a large outbred population.