The use of the routine method for inactivation of the inductor virus by acidification of interferon to pH 2.0 resulted in a significant decrease in the antiviral activity of pig leukocytic interferon, since the preparation was highly sensitive to changes in the pH values. The use of 0.1 per cent solution of formalin provided complete inactivation of the virus. The antiviral activity of interferon treated with formalin was on an average 5 times higher than that of the preparation incubated at pH 2.5-2.6. Precipitation of pig interferon with polyethylene glycol promoted both increasing of the titers of the antiviral activity of the preparation and elimination of the formalin residues from it. Interferon prepared with this procedure was not toxic in tissue cultures, had no side effects when applied to the eye mucosa and was absolutely harmless when administered to animals. It was shown that inactivation of the inductor virus with formalin was in principle possible in human leukocytic interferon.