The suggestion of a substance antagonistic to interferon has been substantiated by Chany and Brailovsky.1 They have offered the first evidence that a reverse phenomenon of the interference factor exists as a stimulating interaction between different viruses, a concept defined as follows: "when two or more antigenically related or unrelated viruses infect the same cell in a certain chronological order, the inducing agent stimulates the multiplication of the challenge virus." Since adenovirus 12 and Kilham's rat virus do not produce significant amounts of interferon in rat cells, they were used in the study of this hypothetical antagonistic factor. Simultaneous infection of embryonic fibroblasts with each virus increased the virulence of the rat virus for rat embryonic cells.
The cytopathic effect of rat virus appeared earlier in cells doubly infected and was not observed in cells infected with adenovirus alone. In consecutive experiments, the enhancement was preceded by a zone