Molecular and cytogenetic analysis of mutations induced by a variety of genotoxic compounds at the heterozygous thymidine kinase locus in mouse lymphoma cells indicates that in vitro assay does indeed register the range of genetic lesions recently found in a wide variety of human tumors. The types and complexity of the induced lesions are reflected in mutant cell colony phenotype in a compound-specific fashion. These studies point to the use of appropriate in vitro mammalian mutagenesis assays as new model systems for dissecting the genetic lesions important to human carcinogenesis, and as a means of determining the potential for compounds to induce such lesions. (Copyright (c) 1992 Elsevier Science Publishers B.V.)