SUMMARYImmunosuppressive sera have been produced by injecting rabbits with soluble extracts isolated from mouse thymocytes. In vitro properties of these antisera are compared with in vivo immunosuppressive activities. Rabbit antisera to soluble extracts from human lymphocytes have also been prepared. Some of these antisera showed in vitro properties suggesting they would be immunosuppressive in man. The soluble lymphocyte extracts have been examined by immunodiffusion and gel electrophoresis.