A leukocyte migration procedure was utilized to test cellular hypersensitivity of breast cancer patients' leukocytes to autologous and homologous breast cancer tissues and to murine milk containing murine mammary tumor virus (MuMTV). The in vitro responsiveness of the leukocytes and the antigenicity of breast cancer tissues were compared with in vivo prognostically favorable lymphoreticuloendothelial (L-RE) responses seen microscopically at the time of mastectomy and with the results of skin window tests of cellular hypersensitivity. The data suggest that immunogens appear in the in situ phase of the disease and provoke prognostically favorable L-RE responses. These immunogens possess antigenic similarity to some component(s) of MuMTV. Progression of the disease is associated with or preceded by a loss of tissue immunogenicity and/or diminished specific cellular hypersensitivity. The findings are pertinent to investigations of human mammary carcinogenesis and immunoprophylaxis.