Fresh and weathered Prudhoe Bay crude oil as well as residues from its combustion were tested for mutagenicity using strain TA98 in the Ames Salmonella typhimurium/liver microsome test. Because of the toxicity of the oils involved, mutagenic effects were clearly visible only at very low concentrations. All samples were found to be mutagenic, with precipitated plume having the greatest activity followed by the burn residue, the weathered oil, and fresh crude, respectively. The most polar components of the neutral fraction of the samples displayed the greatest mutagenicity.