In the present study, we demonstrated the cytotoxic effect of a dimethylarsenic compound in seaweed, (R)-(2',3'-dihydroxypropyl) 5- deoxy-5-dimethylarsinoyl-beta-D-riboside, namely arsenosugar (AsSug), on mammalian cells, murine macrophages, in comparison with that of an inorganic arsenical, arsenite, in vitro. More than 99.5% pure AsSug was synthesized. Arsenite was strongly and equally toxic to both