INTRODUCTION Mammalian cell culture systems provide a valuable tool for assessing the genetic hazards of a variety of potentially Imltagenic agents. The CHO/HGPRT mutation assay was designed to select for Imltant cells which have become resistant to the purine analogue 6-thioguanine as a result of mutation at the X-chrorrosome-linked hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus of Chinese hamster ovary (CHO) cells. This system has been demonstrated to be sensitive to the Imltagenic action of a variety of chemicals (1).