Polyclonal antilymphocyte globulins are obtained by injecting animals, usually horses, with human lymphoid cells, such as B cell lymphoblasts, peripheral T cell lymphocytes, or thymus lymphocytes and then harvesting and processing the immune sera to obtain purified globulin. The principal mechanism of action for these agents are complement-mediated lysis of lymphocytes, uptake of lymphocytes by the reticuloendothelial system, and masking of lymphocyte cell surface receptors.