The expression of multiple recessive genes by aberrant mitotic lesions plays a major part in carcinogenesis. These lesions include intragenic mutations as well as chromosomal lesions. An in vitro model for studying carcinogenesis should respond to all these lesions. Mutagenesis studies that target hemizygous loci may not be useful in studying chromosomal mechanisms because lesions incorporating essential genes already missing on the inactive, homologous chromosome may be lethal to the cell. Cells heterozygous at the selectable gene may survive. Using L5178Y mouse cells, we compared the mutagenic responses at the heterozygous tk and hemizygous hprt loci to four chemicals-benzidine dihydrochloride, diglycidylresorcinol ether, nitrofen and p-benzoquinone dioxime. None of the compounds induced clear positive responses at the hprt locus. In contrast, all the compounds induced clear or marginal mutagenic responses at the tk locus. These data are consistent with the expectation that heterozygous loci can detect lesions that are refractory to hemizygous loci.