The purpose of this investigation has bun to determine whether or not thalidomide has any effect as an immunosuppressant. The isomatibody response of rabbits to erythrocytic antigens was tested in groups of rabbiu rueiving varying dosages of thalidomide. The effect of this drug was also tested in vitro on cell cultures of antibody-producing rabbit popliteal lymph node cells. A definite immunosuppressive effect was noted in vivo, while, no effect on antibody synthesis was detected in vitro. The significance of thest findings is discussed.