An immunosuppressive factor was isolated and partially purified from a fraction of the serum of acute thermally traumatized patients which contained primarily albumins. The factor proved to be either a small protein or a peptide of molecular weight less than 10,000. It inhibited migration of peripheral blood leukocytes of thermally traumatized patients, and of guinea pig peritoneal macrophages. In the presence of the serum, it caused the lysis of peripheral lymphocytes from thermally traumatized patients, and depressed mitogenic stimulation of normal human peripheral lymphocytes. It had no effect on migration, and did not lyse peripheral blood lymphocytes of healthy subjects.