Simple modifications of the Ames assay were evaluated for detecting mutagenicity of petroleum samples which are incompatible with the aqueous medium due to insolubility. Five petroleum samples were assayed using the standard plate incorporation Ames assay protocol. Results of these assays were invalid due to insolubility or separation of sample in the top agar layer. Solubilization of tar samples in DMSD was achieved by sonication and emulsions of liquid samples were prepared by vortexing. Such solubilized/emulsified samples also separated from the top agar overplay in a standard plate incorporation assay. Preincubation of sample and target organism in the presence of sonication (tar samples) or end-over-end rotation (liquid samples) in solutions/emulsions, permitted detection of mutagenicity of two samples and enhanced toxicity of a third. A sixth sample, tested as a gas in glass chambers, was found to be toxic but not mutagenic at the doses tested.