Using the human colon adenocarcinoma-derived cell line Caco-2, we investigated the possible role of the Ca2+-sensing receptor (CaR) in mediating effects of extracellular Ca2+ on cellular proliferation. Caco-2 cells respond to low ambient [Ca2+]o by activation of the protein kinase C-signaling pathway, leading to upregulation of c-myc mRNA expression and thereby, finally, to alleviation from the G1/S phase control of the cell cycle. This proliferative response can be reverted by activation of the CaR either through raising [Ca2+]o or, respectively, by using the CaR agonist Gd3+ as a substitute for Ca2+. The inhibitory effect of [Ca2+]o on cell replication exhibits saturation kinetics (IC50 = 0.045 mM), indicating the existence of a highly sensitive CaR operating at low ambient [Ca2+]o. Specific immunostaining revealed the presence of CaR-positive cells in the crypt epithelium of normal human colonic mucosa as well as in glandular (i.e., differentiated structures) of carcinomatous lesions. This could provide a rationale for use of calcium supplements for intervention in early phases of colon tumorigenesis.