A burned rabbit's model (20% TBSA) was used in this study. The sera taken from the burned rabbits without bacteremia and sepsis showed immune suppressive effects, especially the sera obtained 4 days after injury. After being separated by ultrafiltration, the burned sera were divided into three groups. The ultrafiltrating substances with molecular weight below 10 Kd or between 10 to 30 Kd were found to reduce the production of IL-2 and the lymphocyte blastogensis stimulated by Con A. The substances with molecular weight greater than 30 Kd also showed profound immune suppressive action on lymphocyte. No obviously abnormal protein was found in the burned serum analysed with SDS electrophoresis. It is concluded that the immune suppressive substances in the burned serum were characterised by complexity in components and a wide range of molecular weight.