We have developed an immunocytochemical technique to visualize the receptor for 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] in cryostat sections of human breast tumors and normal human breast tissue utilizing a monoclonal antibody (9A7 gamma) to chick intestinal receptor which recognizes mammalian 1,25(OH)2D3 receptor. Specific staining was observed in the nuclei of tumor cells. Previous studies by our group have shown that a high proportion of breast tumors bind radiolabeled 1,25(OH)2D3 and we have confirmed this, demonstrating immunocytochemical 1,25(OH)2D3 receptor in 43 of 55 (78%) of breast carcinomas. No correlation with the presence of immunostainable estrogen receptor was found in these breast cancer specimens. Sections of normal breast showed immunoreactivity in the nuclei of epithelial cells of the lobules and ducts. Our results demonstrate that the receptor for 1,25(OH)2D3 resides predominantly in the nucleus of breast carcinoma cells. The reason for its prominent expression in breast cancers is not yet known but may be related to growth regulation.