After heating at 37° in minimal Eagle's medium, the alternating polyriboadenylic-polyribouridylic acid [poly r(A-U)] became much more active in producing cellular resistance to virus infection and interferon production in four different cell lines (human skin fibroblasts, mouse embryo fibroblasts, mouse L 929 cells and rabbit kidney (RK 13) cells). The thermal activating effect was neutralized by pre-treatment of the cells with DEAE-dextran, which increased the antiviral activity of unheated and decreased the activity of heated poly r(A-U) to the same level. Under all conditions tested, the degree of cellular resistance to virus infection closely paralleled the amounts of interferon produced suggesting that interferon production is responsible for the antiviral resistance produced by poly r(A-U).