1,25-(OH)2-Vitamin D3, the active metabolite of vitamin D, is a secosteroid hormone with known differentiating activity in leukemic cells. Studies have demonstrated the presence of vitamin D receptors (VDR) in a wide range of tissues and cell types. Antiproliferative activity of 1,25-(OH)2-vitamin D3 has been documented in osteosarcoma, melanoma, colon carcinoma, and breast carcinoma cells. This study was designed to analyze vitamin D receptor level in breast cancer cells as a marker of differentiation and as a predictor of growth inhibition by 1,25-(OH)2-vitamin D3. VDR messenger RNA was found to be present in relatively high levels in well-differentiated cells and in low levels in poorly differentiated cells. All cell lines had detectable VDR mRNA. Radiolabeled ligand binding assay showed a similar pattern. MCF-7 and T47D cells, which express VDR at moderate levels, showed significant growth inhibition by 10(-9) M1,25-(OH)2-vitamin D3 (p < 0.05). MDA-MB-231 cells, which have very low levels of VDR, demonstrated no growth inhibition by 1,25-(OH)2-vitamin D3 at concentrations up to 10(-6) M. Based on these results it can be stated that VDR expression is lost with de-differentiation and that receptor is essential for the antiproliferative response to 1,25-(OH)2-vitamin D3.