The reaction of the host against the grafted tissue in mouse to rat or rat to mouse transplantations corresponds to the most severe reaction observed in homoio- or interracial transplantations. The consider* able variation in the results in individual cases observed in the latter kinds of transplantation is lacking liter heterotransplantation between mouse and rat; here variations are almost limited to the length of time during which preservation of cartilage and perichondrium is observed. Regenerative processes are entirely or almost entirely lacking in these heterogenous transplants. The destruction of the graft is largely due to the unfavorable effect of the body fluids of the host, which does not permit the establishment of a satisfactory circulation around the transplant at an early period following transplantation and may act also otherwise as toxic substances.