Effect of macrogard as an immunostimulator at 1 g/kg feed was assessed on some growth performance and efficacy of a bivalent streptococcosis/lactococcosis vaccine in rainbow trout weighing 1.5 g for a period of 40 days at 12-13°C. Fish were vaccinated via bath route for 2 min and the efficacy of vaccine was evaluated based on the LC₅₀ detection using a virulent strain of Lactococcus garvieae at 1 x10⁷ cfu/ml for10 min. The obtained results showed both vaccine and vaccine plus macrogard groups gave significantly higher weight gain and specific growth rate than unvaccinated fish fed with feed without macrogard (control group) (P<0.05). Also, feed conversion ratio in control group was significantly higher than other two groups (P<0.05). The survival in vaccine and vaccine plus macrogard  groups were 95 and 85 percent, respectively 40 days post-vaccination compare to 75 percent survival in control fish (P<0.05). In addition the relative survival percent in vaccine and vaccine plus macrogard fish were 82.3 and 74.6, respectively (P<0.05). These results shows vaccination of trout could improve the fish growth performance; and use of macrogard in fish feed was able to enhance the efficacy of streptooccosis/lactococcosis vaccine.