title: Erratum.
abstract: In the 21 December response by Bruce N. Ames and Lois S. Gold (Letters, p. 1645) to the letter by Frederica P. Perera (p. 1644), the last sentence of the third paragraph in column three should have read, "Both natural arsenic in water and natural radon in indoor air are present at high levels at some locations and were long neglected, while major efforts were put into miniscule amounts of industrial pollutants." In the 4 January response by Bruce N. Ames and Lois S. Gold (Letters, p. 12) to the letter by David P. Rall (p. 10), reference 11 should have read, "D. G. Hoel, J. K. Haseman, M. D. Hogan, J. Huff, E. E. McConnell, Carcinogenesis 9, 2045 (1988)"; reference 14 should have read, "E. Marshall, Science 250, 900 (1990); R. Doll, Eur. J. Cancer 26, 500 (1990); C. Hill, E. Benhamm, F. Doyon, Lancet 336, 1262 (1990); S. Freni, ibid., p. 1263."