Abstract: The group of inventions is related to medicine, namely infectious diseases, as well as pharmacology, the pharmaceutical industry and biotechnology and provides pegylated interferon lambda having antiviral activity against hepatotropic viruses, wherein said agent has oral bioavailability at least 10%, which ensures high concentration of the agent in the liver, bypassing the systemic circulation, reduces systemic side effects and increases the compliance with therapy, and wherein the method for preparation of the agent comprises irradiating a solution of polyethylene glycol with a molecular weight of from about 400 to about 5,000 Da with ionizing radiation in a dose of 1 to 5 Mrad, mixing the resulting solution with an interferon lambda solution, freezing and cooling of the resulting mixture to a temperature of ?10° C. or below, and the irradiating of the frozen and cooled mixture with ionizing radiation in a dose of 0.1 to 0.5 Mrad.