Cells of a breast adenocarcinoma cell line, CAMA-IEe, were found to contain a moderate concentration of 1, 25-dihydroxyvitamin D3[1, 25(OH)2D3] receptors. The receptors' dissociation constant calculated by Scatchard analysis was 4.1a10−10M. These receptors sedimented at around 4.0 S position on a linear 5–20% sucrose density gradient and exhibited an apparent molecular weight of about 46 kDa. Receptor concentration was found to be highest in the early stages of cell growth, decreased continuously when cells were actively growing, and reached the lowest level at confluence. Suppression of cell growth, of DNA synthesis, and of the phosphorylation of retinoblastoma protein by 1, 25(OH)2D3 at concentrations about 10−7M were observed. Treatment with 10−7M 1, 25(OH)2D3 for 12 hr or longer caused elevation of cytosolic free calcium in CAMA-IEe cells. These results show that CAMA-IEe cell growth is suppressed by 1, 25(OH)2D3 and that the suppression is possibly mediated through calcium mobilization.