Cockayne syndrome (CS) is an autosomal recessive disorder with dwarfism, mental retardation, sun sensitivity and a variety of other features. Cultured CS cells are hypersensitive to ultraviolet (UV) light, and following UV irradiation, CS cells are unable to restore RNA synthesis rates to normal levels. This has been attributed to a specific deficiency in CS cells in the ability to repair damage in actively transcribed regions of DNA at the rapid rate seen in normal cells. We have used the failure of recovery of RNA synthesis, following UV irradiation of CS cells, in a complementation test. Cells of different CS donors are fused. Restoration of normal RNA synthesis rates in UV-irradiated heterodikaryons indicates that the donors are in different complementation groups, whereas a failure to effect this recovery implies that they are in the same group. In an analysis of cell strains from 22 CS donors from several countries and different racial groups, we have assigned five cell strains to the CS-A group and the remaining 17 to CS-B. No obvious racial, clinical or cellular distinctions could be made between individuals in the two groups. Our analysis will assist the identification of mutations in the recently cloned CSA and CSB genes and the study of structure-function relationships.