In this study the effects of Silybum. marianum extract on some immune responses and hematological parameters of the common carp (Cyprinus carpio) were investigated. A total of 280 fish weighing 65.12 ± 8.22 g were randomly divided into four equal groups. Group1 was immunized with Aeromonas hydrophila bacteria and fed with a diet containing 0.5% S. marianum extract. Group 2 was non-immunized and fed with a diet containing 0.5% S. marianum. Group 3 was immunized with A. hydrophila and was fed with a S. marianum-free diet. Group 4was neither immunized nor fed with S. marianum. Blood samples were taken every 10 days for 40 days. Sera samples were analysed for lysozyme activity, serum bactericidal activity, alternative complement activity, total protein, IgM concentration and anti-A. hydrophila antibodies. Blood samples were also used for hematological parameters, PCV, Hb, WBC, RBC, MCV, MCH, and MCHC. 30 fish in triplicates were challenged with A. hydrophila. The results showed that the values for WBC, PCV, total protein, IgM levels, and lysozyme activity were significantly increased in the serum of S. marianum-treated fish (p<0.05). No significant differences were recorded in anti-A. hydrophila antibody levels, complement activity, or other hematological parameters in S. marianum-treated fish compared to controls (p>0.05). The RPS and serum bactericidal activity was only increased in the non-immunized, S. marianum-fed group when compared to non-immunized, S. marianum-free group. This study indicates that oral administration of S. marianum enhances some non-specific immune responses in C. carpio, and therefore it can be recommended as an herbal immunostimulant in fish.