Skin fibroblasts from patients with cockayne syndrome (CS cell) exhibited marked ultraviolet (UV) sensitivity as measured by colony-forming ability. Further, recovery of semiconservative DNA synthesis following UV irradiation was absent in CS cells, as it is in xeroderma pigmentosum cells. We found that the rate of semiconservative DNA synthesis measured at 12 h after 12 J/m2 of UV irradiation had recovered to nearly normal levels in binuclear cells obtained by the fusion of CS strains CS3BE (GM 1856) and CS7SE (GM1428) and of CS3BE (GM1856) and CS1BE (GM1629), but not of C57SE (GM1428) and CS1BE (GM1629). These results indicate that there are at least two genetic complementation groups in CS.