AbstractRecent studies have shown that antibody in the serum of patients with carcinoma of the breast reacts with two distinct antigens obtained from breast cancer tissue. Hence, there are two antibodies. The first antibody reacts against the Fc fragment of immunoglobulin and is also found in the sera of patients with benign breast disease and certain inflammatory lesions and, therefore, does not seem to have any significant meaning for the patient with carcinoma of the breast. The second antibody is directed against the Fab fragment of human immunoglobulin that is found in breast cancer tissue. Recent experiments have shown that in immunodiffusion tests, this autoantibody reacts not with normal IgG(Fab), but rather with heat‐aggregated IgG(Fab). It is thought that tumor‐associated antibodies react with antigens on the tumor cell surface. Intracellular enzymes then proceed to fragment the attached antibody, leaving the Fab fragments attached on the cell surface. With intact immunosurveillance, the host appreciates this altered immunoglobulin (Fab). It is suggested that the autoantibody then formed against this fragment reacts with the fragment attached to the tumor cell, thereby allowing the tumor cell to be destroyed. Clinical data supporting this hypothesis are derived from the fact that 9 patients having this autoantibody to the Fab fragment are alive at 1 year after their carcinoma of the bre