We demonstrate in this study the cytotoxic effects of inorganic arsenicals, arsenite and arsenate, and organic arsenic compounds, monomethylarsonic acid (MAA), dimethylarsinic acid (DMAA), and trimethylarsine oxide (TMAO), which are metabolites of inorganic arsenicals in human bodies, using murine macrophages in vitro. Inorganic arsenicals, both arsenite and arsenate, are strongly toxic to macrophages, and the concentration that decreased the number of surviving cells to 50% of that in untreated controls (IC50) was 5 or 500 microM, respectively. These inorganic arsenicals mainly caused necrotic cell death with partially apoptotic cell death; about 80% of dead cells were necrotic, and 20% were apoptotic. The inorganic arsenicals also induced marked release of an inflammatory cytokine, tumor necrosis factor alpha (TNF alpha), at cytotoxic doses. This strong cytotoxicity of an inorganic arsenical, arsenite, might be mediated via active oxygen and protease activation because it was inhibited by the addition of some antioxidant reagents, such as superoxide dismutase (SOD), catalase, and GSH, or by a peptide inhibitor of interleukin-1 beta-converting enzyme (ICE). It is likely that these immunotoxic effects of inorganic arsenicals may evoke both immunosuppression and inflammation, and they may be central factors causing carcinogenesis and severe inflammatory responses, such as hepatomegaly and splenomegaly, in chronic arsenicosis patients who daily ingested arsenic-contaminated well water. In contrast, the cytotoxic effects of methylated arsenic compounds were lower than those of inorganic arsenicals. The IC50 value of DMAA was about 5 mM, and MAA and TMAO had no toxicity even at concentrations over 10 mM. Additionally, these methylated chemicals suppressed the TNFalpha release from macrophages. DMAA induced mainly apoptotic cell death in macrophages as indicated by cellular morphological changes, condensed nuclei, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL), and DNA fragmentation. However, the cytotoxicity of DMAA might be induced via a different mechanism from that of inorganic arsenicals because it was not abolished by the additions of SOD, catalase, or ICE inhibitor. Conversely, GSH enhanced the toxicity of DMAA. These data suggest that methylation of inorganic arsenicals in mammals plays an important role in suppression of both severe immunosuppression and inflammatory responses caused by inorganic arsenicals.