Skin fibroblasts from a 15-year-old-male patient, who was diagnosed as xeroderma pigmentosum (XP), were cultured to determine the excision repair ability and the complementation group, and the results obtained were as follows: Rates of unscheduled DNA synthesis, DNA single strand breaks formation and excision of pyrimidine dimers produced by UV-light showed that these cells have a residual DNA repair ability of intermediate level between normal and XP group A (XP12R0) cells, having, the lowest residual repair level of all the complementation group in XP. Three XP cell strains were used in complementation study. Hybrid binuclear cells were formed by fusing this cell strain with one of these three cell strains. Unscheduled DNA synthesis in bunuclear cells showed that complementation was not occurred only in binuclear cells formed with XP2BE. From this, the complementation group of XP1SE was revealed to be group C. This cell strain was tentatively named as "XP1SE" after the recommendation of the nomenclature of the XP cell strains by Cleaver and Bootsma (1975).