DEAE-dextran has commonly been found to more effectively enhance transfection when mixed with the virus nucleic acid before cell inoculation (mixed-inoculum method) than when it is used to pretreat the cells before inoculation (Pagano & Vaheri, 1965; Bachrach, 1966; Koch, Quintrell & Bishop, 1966; Pagano, McCutchan & Vaheri, 1967; Tovell & Colter, 1967). We have found, however, that the cells are only transiently sensitized to transfection by pretreatment, and that this method, optimally employed, can give much greater enhancement than the mixed-inoculum method. Part of these results have appeared in abstract form (Al-Moslih & Dubes, 1972).