High voltage electric pulses, 1000 V/cm, were found to induce cell fusion efficiently when delivered to cells growing in a monolayer culture. The maximum yield of fused cells was about 30% of cells remaining after treatment. Colonies of interspecies hybrid cells between mouse and others also appeared with a frequency of 3 X 10(-4) - 2 X 10(-5) after culturing fused cells in a selection medium that permits the growth of only hybrid cells. This method of electrofusion was applied for complementation analysis of DNA repair-deficient xeroderma pigmentosum cells. Efficient cell fusion was also observed with these human diploid fibroblasts and the resulting heterodikaryons showed a recovery of ultraviolet light-induced unscheduled DNA synthesis to the same extent as in normal cells.