Effect of 4-nonylphenol on hepatosomatic, gonadosomatic indicators, and changes plasma IgM immunoglobulin and lysozyme activity were studied in immature Koi carp. In order to, fish for over three week received intraperitoneal injection of 10, 50 and100 micro gram per gram of 4-nonylphenol and 2 micro gram per gram of 17-b -estradiol per body weight. Fish samples were taken on days 0 and 22. The concentration of IgM immunoglobulin was measured by proposed method of Siwicki and Anderson and lysozyme activity was measured by ELISA. Liver Index (HSI) and o.