The by-product from vinyl chloride production, EDC-tar, is a complex mixture of mainly short-chained chlorinated aliphatic hydrocarbons. This mixture has been tested for mutagenicity by means of Ames' Salmonella/mammalian microsome method. Since most of the components in the tar are poorly soluble in water, three agents were used as solvents or emulsifier: ethanol, DMSO, and Tween 80. The results with all these agents showed that EDC-tar contains direct as well as indirect mutagenic constitutents. It could be concluded that the mutagenic effect observed in the test could not be due to any significant extent to one of the main components, ethylene dichloride (1,2-dichloroethane). This substance showed a weak mutagenic effect, but only at higher concentrations than could be available in the highest concentration tested of the tar. Although the microsomal system enhanced the mutagenicity both of the EDC-tar and of 1,2-dichloroethane, this enhancement was dependent on NADPH in the case of EDC-tar but independent of NADPH with 1,2-dichloroethane. The Salmonella/mammalian microsome method seems to be a suitable tool for both mutagenicity screening of complex chemical mixtures and identification of mutagenic constituents in such mixtures.