The L5178Y mouse lymphoma assay has been widely used in short-term mutagenicity testing. Research into the types of genetic damage detected at the thymidine kinase locus indicates that the assay may be capable of evaluating not only the potential gene mutagenicity but also the clastogenicity of a test chemical. The ability to distinguish between the two types of genetic damage is based upon the observation that the mutants can be separated by colony size, using an automatic colony counter, into omega, small colonies and lambda, large colonies. In addition omega mutants appear to represent chromosomal alterations to that chromosome 11 carrying the TK locus, while lambda mutants may represent smaller-scale, single-gene mutations. The process of evaluating the hypothesis includes the testing of agents of known clastogenicity and an evaluation of the relative proportions of the omega and lambda mutant frequencies. Several compounds known to be clastogens have already been shown to induce a significant proportion of omega mutants.