The treatments of fusion-elution and permeabilization-cell extracts were used to study the characteristics of xeroderma pigmentosum (XP) factors that complement the defects of XP cells in the repair of DNA damages induced by UV-irradiation. Unscheduled DNA synthesis in XP1EH (complementation group A) cells exposed to 10 J/m2 UV-light, permeabilized and repair-labeled with 3H-dTTP, was remarkably increased by the treatment with cell extracts of HeLa or XP3KA (complementation group C) cells. Single strand breaks of XP A or C cells, fused with HeLa cells, was produced to a similar extent of those in HeLa cells irradiated with the same dose of UV-light. These results indicate that XO A and C cells are defective in different genes, and that the XP A and XP C factors are present in at least two fold excess for the maximal activity of excision repair in HeLa cells. The rate of single strand break of DNA in XP and HeLa cells was measured as a function of time after UV-irradiation. The maximum elution rate in all repair-defective XP cell lines appeared at 1-2 hr and then gradually reduced as the time lapsed, while those of HeLa cells appeared at 0-1 hr. The above results strongly suggest that the defects of XP cells might be underlain on the problems in the factors provoking conformational changes for the accessibility of the repair enzyme to the damaged sites.