A mutant Chinese hamster cell line was selected from wild-type CHW cells in 30 µg/ml 8-azaguanine, after exposure for 2 h to 10–3 M methyl methanesulphonate. This line was subsequently cloned and one clone, isolated in 10 µg/ml 8-azaguanine (CHW-1102), was selected for further study. CHW-1102 showed stability of resistance to 8-azaguanine after six months of growth in nonselective medium and an LD50 to 8-azaguanine of between 10 and 20 µg/ml, compared to 2.5 µg/ml for the wild-type CHW cell line. The doubling time in culture was found to be 13 h, compared to a doubling time of 12 h for CHW. Biochemical studies showed that the mutant cell line CHW-1102 had a specific activity of HPRT of about 1–2 nmol/mg protein per hour compared to a specific activity of the wild type of 200 nmol/mg protein per hour. The spontaneous reversion rate from 8-azaguanine resistance to 8-azaguanine sensitivity was 4.23 × 10–8/locus per generation. Both the parental line (CHW) and the mutant (CHW-1102) have a modal chromosome number of 22 and a relatively stable karyotype. Both lines show some chromosome rearrangement and, in particular, the presence of one long acrocentric marker chromosome, which consists largely of material from chromosomes 6 and 8. Much of the distal part of the long arm of the X chromosome, which is heterochromatic, has been lost in both CHW and CHW-1102. So far as can be determined, no autosomal material, except perhaps the paracentromeric region of chromosome 6, is missing in CHW-1102, although even this material may be represented by a small marker chromosome M2. Both cell lines show characteristics that are of value for the study of mammalian cell genetics, and CHW-1102 represents a useful addition to the stock of mutant mammalian cell lines.