In the past few years there has been an increase in Poland of IHN virus isolation on rainbow trout farms. Diagnostic studies using biological, immunoassay (ELISA), and molecular (PCR) methods have been performed systematically. Simultaneous intensive study has also demonstrated that the IHN virus has an immunosuppressive effect that causes decreased macrophage and lymphocyte activity in rainbow trout. One method to limit losses caused by the IHN virus is to stimulate the innate antiviral immune response in spawners. The aim of the study was to determine the impact of a new immunostimulator, Bioimmuno II (IRS Olsztyn), which was administered with the feed for 14 days, on the innate cellular and humoral defense mechanisms of rainbow trout spawners. The experiment was performed on 20 female and 20 male rainbow trout. Immunoassays indicated that Bioimmuno II stimulates the innate cellular and humoral mechanisms in females and males. The increased activity of the macrophages and lymphocytes is particularly important, while increased levels of interferon is a very significant element of innate antiviral immunity. The results obtained suggest that Bioimmuno II can be useful in activating the innate antiviral immunity to IHN in rainbow trout spawners.