Ultraviolet-inactivated Sendai virus used in conjunction with a selective system reproducibly gives high yields of viable hybrid cell lines. With two different crosses, the frequency of hybrid colonies obtained after virus treatment was found to be 100 times greater than the frequency of spontaneous hybrids, and this increase was found to show little variation between 33 and 1000 hemagglutinating units of virus. No differences have been found between the properties of hybrids obtained after Sendai treatment and those obtained from untreated mixed cultures of parental cells.