The function of Ca(2+) and the calcium sensing receptor (CaSR) in breast epithelium and its relationship to mammary carcinogenesis is poorly understood. In this study, we determined the function of this ligand receptor system in regulating the biologic properties of the estrogen receptor-positive MCF-7 and the estrogen receptor-negative MDA-MB-435 human breast cancer cells. Physiologic concentration of extracellular Ca(2+) (by comparison to cells cultured in control low Ca(2+) medium) down-modulated cellular proliferation, cellular invasion and growth in soft agarose in both of these cell lines. Physiologic concentration of extracellular Ca(2+) also down-modulated the expression of the anti-apoptotic protein survivin, survivin gene transcriptional activity, survivin mRNA expression and promoted a cytotoxic response to paclitaxel. These responses to extracellular Ca(2+) were found to require CaSR expression because knocking down CaSR expression in these cells abrogated the cellular responses to extracellular Ca(2+). Each cell line was found to contain small subpopulations that did not express CaSR but expressed a higher level of survivin. These subpopulations were relatively resistant to paclitaxel by comparison to cells that expressed CaSR with a lower level of survivin expression. It is concluded that extracellular Ca(2+) and CaSR may constitute a robust ligand-receptor system in regulating the biologic phenotype of breast epithelial cells and loss of CaSR expression may promote malignancy and resistance to cytotoxic drugs.