The effects of tri- and penta-valent arsenic compounds on sister-chromatid exchange (SCE) in both G0 and 5-bromodeoxyuridine (BrdU) substituted chromosomes was investigated. ¿Of 4 donors whose lymphocytes were exposed to sodium arsenate for 2 cell cycles 1 showed an increase, 2 a decrease, and 1 neither an increase or decrease over baseline levels. Exposure to sodium arsenite produced a similar result with 2 of 4 donors showing a significant increase in SCE. G0 lymphocytes exposed to either sodium arsenate or sodium arsenite did not show increased SCE. These results indicate that although arsenic can induce SCEs in human lymphocytes, there is considerable variation in SCE response among individuals. Furthermore, the data suggest that the increased SCEs seen in patients exposed to arsenic may be due to the patient's condition and not arsenic exposure.