Vasodilating drugs, inhibitors of cAMP phosphodiesterase activity (theophylline, caffeine, theobromine, etc.) and other vascular drugs (papaverin, dibasol, no-spa, etc.) were found in experiments in vivo to be capable of inducing interferon the peak of whose production was determined in the blood of animals 24 hours after drug administration (the observation period). The interferon production is critically dependent upon the dose and routes of administration of the drugs. The interferon induced by vasodilating drugs belongs to interferon of the 1st species. It is assumed that both endothelial cells of vessel walls and lymphocytes may take part in interferon production.