The L5178Y/TK+/- mouse lymphoma assay is utilized to quantitate the induction of thymidine kinase (TK)-deficient mutants. TK-heterozygous cells are treated with the test compound, newly induced mutants are allowed to express, and mutants are selected with trifluorothymidine (TFT). Mutant colonies detected in the assay can be classified by colony size as large (> than approximately 0.6 mm) and small (< than approximately 0.6 mm). An extensive analysis of these two classes of mutants has led to the hypothesis that the two types of mutants result from different degrees of genetic damage and that the assay can be used to detect and discriminate between chemicals acting as point mutagens (inducing significant numbers of large colonies) and/or clastogens (inducing significant numbers of small colonies). This raises the question as to how the small-colony TK mutant frequency would be related to the gross aberration frequency obtained by standard microscopic technique. Preliminary studies with a very small number of chemicals indicated that there might be a simple mathematical comparison between the two endpoints.