We tested whether recognition of bone-related peptides regulates intracellular Ca2+ concentration ([Ca2+]i) of giant cell tumor of bone (GCT). [Ca2+]i was measured in single cells by fura 2 fluorometry. GCT cells were sensitive to bone sialoprotein-II (BSP-II), osteopontin (OPN), and related fragments. Responses consisted of a prompt increase of [Ca2+]i, mostly transient, with a peak followed by a rapid return toward baseline. Responses were not mimicked by bovine plasma fibronectin. Sensitivity of GCT cells to bone peptides was specific, since BALB/3T3 fibroblasts and U-937 histiocytic lymphoma cells with monocytic phenotype failed to respond to BSP-II and OPN fragments. GRGDSP synthetic esapeptide, carrying the Arg-Gly-Asp adhesive motif, and GRGESP (Asp replaced by Glu), but not the GRADSP (Gly replaced by Ala), were active in inducing [Ca2+]i transients as well. Responses were observed also in cells treated with the BSP-II 1C fragment, lacking any known adhesive sequence, indicating that the active peptides inducing [Ca2+]i increments may be multiple. Sensitivity to extracellular matrix peptides was present in a variable fraction of the cells and was downregulated on long-term culture. The mechanism inducing [Ca2+]i elevations was mostly related to Ca2+ release from thapsigargin-sensitive intracellular pools.