Receptors for 1,25 dihydroxyvitamin D3 [1,25-(OH)2D3] have been reported in breast tissue; however, the presence of multiple binding sites and limited availability of human tumor tissue have precluded complete biochemical characterization of the receptor in breast cancer. In the present study, binding proteins for 1,25-(OH)2D3 in breast tumor tissue were analyzed using a rat model of breast cancer. Breast tumors were induced in adult female rats with the carcinogen nitrosomethylurea. Such tumors previously have been shown to possess high levels of estrogen receptors and are estrogen dependent. Binding proteins for 1,25-(OH)2D3 in 0.3 M KCl extracts of tumor tissue were analyzed on sucrose density gradients. Two binding proteins were detected: one sedimenting at 5-6 S representing binding of 1,25-(OH)2D3 to the 25 hydroxyvitamin D3 (25OHD3) binding protein and a second moiety sedimenting, like the rat intestinal receptor, at 3.3 S. Binding of the dihydroxy metabolite to the faster sedimenting protein could be eliminated by inclusion of radioinert 25OHD3 in the incubation medium. Receptor content of rat breast tumor was investigated using an hydroxylapatite assay by incubating tumor extracts with a saturating concentration of 1,25-(OH)2-[3H]D3, plus unlabeled 25OHD3 to eliminate binding of the hormone to the 5-6 S species. Scatchard analysis of 1,25-(OH)2D3 binding to the tumor extracts yielded an apparent dissociation constant (Kd) of 0.33 nM. In summary, breast tumors induced in rats by nitrosomethylurea were shown to contain high affinity 1,25-(OH)2D3 receptors with properties very similar to those reported for the receptor in other mammalian target organs. The presence of receptors for 1,25-(OH)2D3 in these rat breast tumors implies that the tissue is potentially responsive to the hormone.