The cloning of a G protein–coupled, extracellular calcium (Ca²⁺_e)-sensing receptor (CaR) from bovine parathyroid provided direct evidence that Ca²⁺_e-sensing can occur through receptor-mediated activation of G proteins and their associated downstream regulators of cellular function. CaR transcripts and protein are present in various tissues of humans and other mammals that are involved in Ca²⁺_e homeostasis, including parathyroid, kidney, and thyroidal C-cells. The present study was performed to determine whether bone marrow cells express the CaR, since cells within the marrow space could be exposed to substantial changes in Ca²⁺_e related to bone turnover. Using DNA and RNA probes from the human parathyroid CaR cDNA, we identified CaR transcripts of 5.2 and 4.0 kilobases by Northern analysis of poly(A⁺) RNA from low-density mononuclear cells isolated from whole human bone marrow that are putatively enriched in marrow progenitor cells, including bone cell precursors. In situ hybridization also identified CaR transcripts in the same cell preparations. Reverse transcription-polymerase chain reaction demonstrated >99% nucleotide identity between transcripts from human bone marrow cells and the corresponding regions of the human CaR cDNA. Antisera specific for several different regions within the extracellular domain of the CaR were reactive with low-density human marrow cells that were either adherent or nonadherent to plastic. About one-third of the adherent, CaR-immunoreactive cells were also positive for alkaline phosphatase, a nonspecific marker of preosteoblasts, osteoblasts, and assorted cells of the colony-forming unit-fibroblast lineage. In addition, a substantial fraction (60%) of low density murine marrow cells cultured for 1 week at 4.8 mM Ca²⁺_e expressed both CaR immunoreactivity and nonspecific esterase, an enzyme expressed by monocyte/macrophages and fibroblasts. Finally, erythroid precursors and megakaryocytes from murine marrow as well as blood platelets expressed abundant CaR immunoreactivity, while peripheral blood erythrocytes and most polymorphonuclear leukocytes did not. These studies indicate that the CaR is present in low-density mononuclear bone marrow cells as well as in cells of several hematopoietic lineages and could potentially play a role in controlling the function of various cell types within the marrow space.