Arsenobetaine, arsenocholine, trimethylarsine oxide and tetramethylarsonium iodide, which are contained in marine fishery products, were examined for their potencies on cell growth inhibition, chromosomal aberration and sister chromatid exchange (SCE). Arsenobetaine, the major water-soluble organic arsenic compound in marine animals, exhibited very low cytotoxicity towards mammalian cells. This compound showed no cell growth inhibition at a concentration of 10 mg cm Ϫ 3 and the cytotoxicity was lower than 1/14 000th of that of sodium arsenite and 1/1600th of that of sodium arsenate towards BALB/c 3T3 cells. The chromosomal aberrations caused by arsenobetaine at a concentration of 10 mg cm Ϫ 3 consisted mainly of chromatid gaps and chromatid breaks, but in this concentration chromosomal breakage owing to its osmotic pressure is likely to be considerable. No SCE was observed at a concentration of 1 mg cm Ϫ 3 . Arsenocholine and trimethylarsine oxide also showed no cell growth inhibited at a concentration of 10 mg cm Ϫ 3 . However, tetramethylarsonium iodide inhibition the growth of BALB/c 3T3 at a concentration of 8 mg cm Ϫ3 . These compounds exhibited a low ability to induce chromosomal aberrations at a concentration range of 2-10 mg cm Ϫ 3 and no SCE was observed at a concentration of 1.0 mg cm Ϫ 3 . These results suggested that the major and minor organic arsenic compounds contained in marine fishery products are much less cytotoxic inorganic arsenic, methy-larsonic acid and dimethylarsinic acid.