The effect of oxidants on Ca 2+ movement in osteoblasts is unclear. In this study, we show that 4,4¢-dithiodipyridine (4,4¢-DTDP), a reactive disulphide that mobilizes Ca 2+ in muscle, induces an increase in cytoplasmic free-Ca 2+ concentrations ([Ca 2+ ] i ) in MG63 human osteosarcoma cells loaded with the Ca 2+ -sensitive dye fura-2. 4,4¢-DTDP acted in a concentrationdependent manner with an EC 50 of 10 lM. Removing extracellular Ca 2+ reduced the Ca 2+ signal by 35%. In Ca 2+ -free medium, the 4,4¢-DTDP-induced [Ca 2+ ] i increase was not changed by depleting store Ca 2+ with 50 lM brefeldin A (a Golgi apparatus permeabilizer), by 2 lM carbonylcyanide m-chlorophenylhydrazone (CCCP, a mitochondrial uncoupler), by 1 lM thapsigargin (an inhibitor of the endoplasmic reticulum Ca 2+ pump) or by 5 lM ryanodine. Ca 2+ signals induced by 4,4¢-DTDP in Ca 2+ -containing medium were not affected by modulation of protein kinase C activity or suppression of phospholipase C activity. However, 4,4¢-DTDP-induced Ca 2+ release was inhibited by a thiolselective reducing reagent, dithiothreitol (0.05-2.5 mM), in a concentration-dependent manner. Collectively, this study shows that 4,4¢-DTDP induced [Ca 2+ ] i increases in human osteosarcoma cells via releasing store Ca 2+ from multiple stores in a manner independent of protein kinase C or phospholipase C activity. The store Ca 2+ release induced by 4,4¢-DTDP appears to be associated with thiol oxidation. Furthermore, overnight incubation with 4,4¢-DTDP inhibited cell activity in a concentrationdependent manner.