The mutagenic and cytotoxic responses of AS52/gpt and CHO-K1/hprt Chinese hamster cell lines were compared using the model mutagens MNNG, ENNG, paraquat, and potassium bromate. The CHO-K1 hprt gene is on the X chromosome and is surrounded by several important housekeeping genes. AS52 cells lack the hprt gene but carry an introduced E. coli gpt gene on an autosome. AS52 cells are expected to detect extended deletions more effectively than CHO-K1 cells, because the loss of some CHO-K1 housekeeping genes, which cannot be compensated for by active genes on a homologous chromosome, could be lethal for that cell line.MNNG induced mutant colonies dose-dependently almost identically in both cell lines. ENNG was also mutagenic in both cell lines, but AS52 cells showed higher sensitivity even though it was expected that both lines would be equally sensitive to ENNG because of its dominant, base-change type of mutagenicity. Paraquat, which is an active oxygen-donating chemical and is expected to induce deletion mutations, induced mutant colonies more intensely in AS52 than in Potassium bromate consistently induced more mutant colonies in AS52 than in CHO-K1 cells. Although AS52 tended to be more susceptible to chemicals that induce deletions, no conclusions can be made regarding the characteristics until further studies using more chemicals with different modes of action are conducted.