Cells of the V79–4 Chinese hamster line were found to have a consistent set of 20 chromosomes. G- and C-band analysis showed that, compared to the standard 22 chromosome set of freshly isolated Chinese hamster cells, the V79–4 chromosomes had various characteristic deletions and rearrangements. However, it is probable that at least one copy of each autosome was still present, and three pairs of autosomes appeared unchanged from those in the freshly isolated cells. The establishment of this karyotype for V79–4 allows mutant sublines to be screened for chromosomal changes associated with the altered phenotype, and an example is given of a radiation-induced mutant lacking HPRT activity that had a clear X-chromosome rearrang