Background: Organic acids and their salts are known as appropriate substitutes in feed for improving the health, growth, and performance of fish. Objectives: The present study was conducted to determine the effects of dietary lactic acid supplementation on immunological factors, hepatic enzyme activity, and plasma proteins in common carp, Cyprinus carpio. The fish were fed the diets mentioned above for 56 days, then their growth performance, humoral immunity, and plasma hepatic enzymes were assessed.Methods: A total of 180 fish (mean weight=25 g) were randomly distributed in twelve tanks (150 L water in each tank) as four treatments, fed diets containing 0, 2.5, 5, and 10 g/kg lactic acid (T0, T1, T2, and T3, respectively).Results: At the end of the feeding trial, T2 showed significantly higher growth performance than T0. Plasma total protein, albumin, and globulin levels of T1-T3 were significantly higher than that of T0. However, plasma protein levels decreased significantly by elevation in dietary lactic acid concentration (10 g/kg). No significant differences were observed in plasma aspartate aminotransferase (AST) and alkaline phosphatase (ALP) activity among the treatments, although alanine aminotransferase (ALT) activity decreased significantly in fish-fed dietary lactic acid supplementation (T1-T3) compared with the control group (T0). All humoral immunity parameters (lysozyme, complement, immunoglobulin, and bactericidal activity) increased significantly in T1-T3 treatments compared to the T0 group.Conclusion: Overall, dietary lactic acid supplementation improves growth performance, humoral immunological parameters, and hepatic health. According to the results, dietary lactic acid (2.5-5 g/kg) is recommended for preparing common carp feed.