Diethylaminoethyl-dextran (DEAE-D) enhanced the infectivity of laryngotracheitis virus (LTV) for chicken kidney (CK) cells when cultures were treated before inoculation with virus and when DEAE-D was present in the inoculum. Infectivity was not increased when cultures were treated after virus had adsorbed to cells; since infection was not synchronized, most of the virus had probably already penetrated the plasma membrane by the time DEAE-D was added. Maximal enhancement occurred when DEAE-D was present in the inoculum. Enhancement of a lesser degree occurred when virus and DEAE-D were mixed, diluted, and inoculated onto cultures. Adsorption of LTV at 37 C as compared to that at 5 C usually yields about a threefold greater number of plaques after a 2-hr adsorption period. However, when DEAE-D was incorporated in the inoculum, greater enhancement occurred at 5 C than at 37 C, and the number of plaques produced at both adsorption temperatures was about equal. Results are compatible with the hypothesis that increased adsorption is a factor in enhancement of infectivity of LTV by DEAE-D.