The reactivities of methyloxoarsine (MAs(III)) and iododimethylarsine (DMAs(III)), two methylated trivalent arsenicals, toward supercoiled phiX174 RFI DNA were assessed using a DNA nicking assay. The induction of DNA damage by these compounds in vitro in human peripheral lymphocytes was assessed using a single-cell gel (SCG, "comet") assay. Both methylated trivalent arsenicals were able to nick and/or completely degrade phiX174 DNA in vitro in 2 h incubations at 37 degrees C (pH 7.4) depending on concentration. MAs(III) was effective at nicking phiX174 DNA at 30 mM; however, at 150 microM DMAs(III), nicking could be observed. Exposure of phiX174 DNA to sodium arsenite (iAs(III); from 1 nM up to 300 mM), sodium arsenate (from 1 microM to 1 M), and the pentavalent arsenicals, monomethylarsonic acid (from 1 microM to 3 M) and dimethylarsinic acid (from 0.1 to 300 mM), did not nick or degrade phiX174 DNA under these conditions. In the SCG assay in human lymphocytes, methylated trivalent arsenicals were much more potent than any other arsenicals that were tested. On the basis of the slopes of the concentration-response curve for the tail moment in the SCG assay, MAs(III) and DMAs(III) were 77 and 386 times more potent than iAs(III), respectively. Because methylated trivalent arsenicals were the only arsenic compounds that were observed to damage naked DNA and required no exogenously added enzymatic or chemical activation systems, they are considered here to be direct-acting forms of arsenic that are genotoxic, though they are not, necessarily, the only genotoxic species of arsenic that could exist.