INTRODUCTION The purpose of this study was to evaluate the mutagenic potential of the test substance@ Petroleum Coking Sample 4-1-140@ using three test systems; the Ames Test@ the Mouse Lymphoma Test@ and the In Vivo Cytogenetics Assay. The Ames Test is a bacterial mutagenicity assay utilizing five selected strains of Salmonella typhimurium. The organisms are exposed to varying concentrations of the test agent both without metabolic activation. The Mouse Lymphoma Test is a mammalian cell assay utilizing the L5178Y mouse lymphoma cell line. The cells are exposed to varying concentrations of the test agent both with and without metabolic activation. The assay measures the mutation frequency at the thymidine kinase locus (TK). The In Vivo Cytogenetics Assay evaluates the mutagenic potential of a test material as manifested by the production of cytogenetic abnormalities such as deletions@ exchanges@ rings@ and breaks in the chromosomes of bone marrow cells from rats receiving the test material.