Estrogen binding protein (EBP) in mammary tumor was studied in DMBA induced rat mammary carcinoma, Shionogi carcinoma 115, Shionogi carcinoma 42 and human mammary tumor by use of (3)H-estradiol. Free and bound estradiol was separated by Sephadex G-25. Specific EBP was calculated 4.5S by sucrose gradient centrifugation. Non-specific EBP was observed in the 3.2S-3.6S region of the ultracentrifugal analysis. Specific EBP was migrated to anodic region of α- or β-position in disc electrophoresis.