This study was performed to investigate the influence of 1,3-1,6-β-D-glucan (Leiber® Beta-S), natural product, on the antibody secreting cells (ASC) and specific antibody levels after immunization of rainbow trout (Oncorhynchus mykiss) with anti-enteric redmouth disease vaccine (AquaVac ERM). Fish were fed pellets containing of 1,3-1,6-β-D-glucan (Leiber® Beta-S) at doses 100 mg and 200 mg per kg of pellets per day. After one week of Leiber® Beta-S supplementation, the fish were immunized by immersion of the vaccine. At 7, 14, 21, 28 and 40 days after immunization, blood and pronephros were taken from 20 fish in each group for immunological study. When analyzed by the ELISPOT assay, Leiber® Beta-S at doses 100 mg and 200 mg per kg of pellets increased the number of specific ASC and specific antibody levels in serum analyzed by the ELISA. The highest numbers of ASC and specific antibody levels at a dose 200 mg per kg of pellets were observed. In conclusion, the results of the present study showed that Leiber® Beta-S increased the effectiveness of anti-enteric redmouth disease vaccine in rainbow trout.