The tumour specific antigens and tumour associated embryonic antigens expressed on chemically induced rat hepatomata and sarcomata have been shown to be different by means of blocking antibody studies. Serum from multiparous female rats contained blocking antibody which protected both tumour and embryo cells from the in vitro cytotoxic effect of lymph node cells from multiparous donors. These sera did not, however, block the cytotoxicity of lymph node cells from tumour immune rats for cells of the immunizing tumour. In addition, the tumour specific rejection antigens and embryonic antigens have been shown to have dissimilar locations in tumour cells.