An inhibitor of virus is demonstrable in the serums of mice infected with Plasmodium berghei. The titer of the inhibitor, detectable within 10 hours after injection of parasitized blood, increases rapidly until 30 to 40 hours when it levels off or decreases slightly before reaching a plateau. The factor that induces production of the antiviral substance is not present in the plasma of the infected blood, and the inhibitor is not detectable in serums of mice injected with nonparasitized mouse erythrocytes. The inhibitor fulfills the essential require ments of an interferon.