The mutagenic activity of tobacco smoke has been further investigated with the plate-incorporation method and a microsuspension technique of the Ames Salmonella assay. The microsuspension test gives a higher response than the conventional plate incorporation test. It is possible to detect environmental tobacco smoke (ETS) in moderately smoky indoor environments by collection of particulate matter with personal low volume samplers followed by particle extraction and mutagenicity testing with the microsuspension assay.