We found decreased cell viability in fibroblast cultures from two patients, one with ring chromosome 4, the other with ring chromosome 15, who had severe somatic retardation. Cell viability was studied with the trypan blue exclusion assay, and in comparison with matched control cells was found to be 72.0 vs. 93.1%, and 79.8 vs 90.2%, respectively. Cloning efficiency of fibroblasts from the patient with ring chromosome 15 was also decreased (12.3 vs. 25.7%). It is suggested that the decreased cell viability is due to a continuous production of hypomodal cells, a process related to the ring structure per se, and perhaps responsible for the severe somatic retardation frequently observed in patients with a ring chromosome.