Mice bearing transplanted lymphoid leukaemia rejected grafts of allogeneic skin in normal manner. Cells fromtheir spleens, collected in progressive stages of the disease, mounted cellular immune reactions as competently as the spleen cells of normal mice. Radiolabelled leukaemia cells accumulated in graft beds These findings were interpreted as showing that malignant cells of this leukaemia may become recruited to take part in the immune response.