Hormone dependency of mammary tumor was studied from the concentration of estrogen binding protein (EBP) by use of (3)H-estradiol. Estrogen dependent rat mammary tumor induced by DMBA was high concentration of EBP, and androgen dependent tumor of Shionogi carcinoma 115 and non-dependent tumor of Shionogi carcinoma 42 were low concentration of EBP. In human mammary tumor the dependent tumor was high and the independent tumor was low concentration of EBP. In case of hormonal dependent breast cancer, EBP was bound above 3μg/mg. protein of estradiol. In case of independent ones, it was below 2μg/mg. protein. Specific EBP was migrated to anodic region of α- or β-position in disc electrophoresis.