Various inorganic compounds of arsenic have been tested for mutagenicity in a variety of test systems ranging in complexity from bacteria to peripheral lymphocytes of exposed human beings. Although a great deal of the data are contradictory, the weight of evidence supports the following conclusions: (1) Arsenic is either inactive or extremely weak for the induction of gene mutations in vitro; (2) Arsenic is clastogenic and induces sister chromatid exchanges (SCE) in a variety of cell types, including human cells, in vitro; trivalent arsenic is approximately an order of magnitude more potent than pentavalent arsenic. (3) Arsenic does not appear to induce chromosome aberrations in vivo in experimental animals; (4) Several studies suggest that human beings exposed to arsenic demonstrate higher frequencies of SCE and chromosomal aberrations in peripheral lymphocytes; the quality of these studies is generally poor.