AbstractKinetic studies of I131‐labeled rat anti‐Walker 256 antibodies are described. The anti‐tumor antibody is a 7S globulin. The presence of tumor diminishes the level of circulating antibody. When the tumor is removed, the antibody level rises to a measurable amount. Ouchterlony in‐gel diffusion studies as well as I131‐globulin uptake data on subcellular tumor fractions indicate that the antigen is probably located on the cell membrane. Whether the antigen studied is a transplantation rather than tumor‐specific type was not determined. It is felt that the techniques described are useful for studying the human patient with autochthonous tumor as well as the cross‐immunoth