The fibroblast strain GM3498 and the lymphoblastoid line GM4408 have been established from the same Bloom's syndrome patient, as confirmed by DNA hybridization with hypervariable probes and isoenzyme analysis. Strain GM3498, but not GM4408, exhibits a high frequency of spontaneous sister chromatid exchange, hypersensitivity to ethyl methanesulphonate and an anomalously low level of DNA ligase I activity. We conclude that GM3498 is representative of Bloom's syndrome, whereas GM4408 is a revertant cell line. These data agree with a model in which a malfunctioning DNA ligase I is the primary cause of the syndrome.