“…The Salmonella assay, in particular, has provided investigators with a simple and inexpensive method whereby organic extracts of airborne particulate matter can be tested for mutagenic activity. This assay, developed nearly two decades ago [Ames et al, 19731, has been used to address several key issues pertaining to air pollution, including the characterization of principal sources of genotoxic emissions [Alfheim et al, 1983;Daisey et al, 1980;Fukino et al, 1982;Goto et al, 1982b;Huisingh et al, 1979;Lewis et al, 1988;Lofroth, 1978;Ohnishi et al, 1980;Matsushita et al, 1986;Mdler and Alfheim, 1983;Mumford and Lewtas, 19821, the assessment of individual atmospheric constituents [see, for example, Graedel et al, 19861, the evaluation of meteorological and atmospheric parameters that influence genotoxicity [Alfheim and Mdler, 1981;Claxton et al, 1990;van Houdt et al, 19871, and the monitoring of industrial and rural areas [Alink et al, 1983;Barale et al, 1989;de Raat et al, 1985;Takedaet al, 1984;Tokiwa et al, 19831. In addition, the Salmonella assay has been used to correlate the mutagenicity of urban air particulate matter with elevated lung cancer mortality rates [Walker et al, 1982;Mumford et al, 19871. A variety of modifications to the standard Salmonella plate-incorporation assay have been reported in recent years, each designed to improve test sensitivity or to accommodate special test considerations. One such modification is the preincubation method first introduced by Yahagi et al…”