It has been estimated that as many as 78% of non-neurologic deaths following trauma are attributable to sepsis and infection related complications. Immune suppression following trauma was thought as the main cause of infection or sepsis after injuries. Many papers have suggested that there were some immune suppressive substances in the serum of burned body. In this study we tried to demonstrate whether there were some similar immune suppressive factors in the serum of blunt injured animals. Thirty rabbits with comminuted fracture of both hind limbs were studied. The serum samples were taken before injury, and 2, 4, 6 days after injury. All sera taken from these days after injury showed immunosuppressive action on lymphocyte proliferation and production of interleukin-2. The most marked suppressive effect among the sera was found in the sera collected 2 days after injury. After being analyzed by SDS-PAGE. an abnormal protein with molecular weight of about 9000 dalton was found in the sera. Furthermore, the abnormal protein was isolated by ultrafiltration with nominal molecular weight limit 10 KD. The results indicated that the abnormal substance had more significant suppressive effects on lymphocyte than other substances with molecular weight greater than 10 KD. This suggested that immunosuppressive effect was also found in the serum from blunt injured animals, and the main cause of immunosuppression of serum was due to the abnormal protein with molecular weight of about 9000 dalton.