The article presents issues related to the problem of applying biocidal substances to a polyethylene film produced in the blow extrusion process in order to extend the functional properties of the film, while maintaining its existing processing parameters. The aim of the research was to determine how the technology of applying a biocidal preparation to the surface of the plastic film will affect the selected properties of the blow polyethylene film. For this purpose, two methods of applying the preparation to a plastic film have been developed. The first method consisted in spraying the biocidal preparation directly during the extrusion process onto the still hot foil surface. The second method was to spray the biocidal preparation on the cooled polyethylene film in a horizontal position. The uniformity of the preparation application, the coefficient of friction and the surface free energy of the film were tested, and the quality and mechanical strength of the welded joint of the surface-modified film were assessed. The geometrical features of the polyethylene films resulting from the course of blow extrusion process were also determined.