Long-term exposure to sodium arsenite (AsO2) promotes the development of various cancers. Paradoxically, arsenic also induces pro-myelomonocytic leukemia cell differentiation, and at higher concentrations, apoptosis. The present study investigated the effects of AsO2 on preosteoclasts. When treated with 2.5-5 μM AsO2, RAW264.7 cells underwent osteoclast differentiation as evidenced by an increase in the number of multinucleate cells expressing tartrate resistant acid phosphatase (TRAP). The appearance of these phenotypic markers was preceded by a low level increase in extracellular production of H2O2 and was prevented by the addition of catalase (4.5 μg/ml), an enzyme that removes H2O2. Only at high concentrations (10-25 μM) of AsO2 was a significant loss of cell viability and a high level increase in H2O2 production (1.5 μM) observed. Apoptosis was blocked by pretreatment with diphenylene iodonium chloride (2 μM), a NAD(P)H-flavoprotein inhibitor, suggesting the involvement of NADPH-oxidase. The data show that AsO2, dose-dependently, stimulates increasing amounts of H2O2 production. Moreover, at concentrations found in tissues of individuals exposed to geochemical AsO2, osteoclasts underwent an H2O2-dependent differentiation. Therefore, chronic exposure to low-level amounts of AsO2 could result in increased bone resorption and contribute to bone related pathologies. © 2006 Elsevier Inc. All rights reserved.