Sodium arsenite, potassium arsenite, and Fowler's solution (arsenic trioxide dissolved in potassium bicarbonate) are equally active in the mouse bone marrow micronucleus assay ( 1 10 mg/kg by IPijection). The natural ore orpment (principally AS2S3) was inactive despite blood levels of arsenic of 300 to 900 ng/mL in treated mice at 24 hr. Sodium arsenite was active in three strains ofmice. It is suggested that the human lung cancer observed among arsenic ore smelters and the skin cancer among people exposed therapeutically to Fowler's solution, have, as their common origin, the genotoxic arsenite ion AsO;-. The difficulty experienced when attempting to demonstrate rodent carcinogenicity for derivatives of arsenic suggests that the bone marrow micronucleus assay may act as a useful assay for potentially carcinogenic arsenic derivatives.