Antilymphocyte sera, either allogeneic or xenogeneic, may retain the antigen used for the immunization of the animal producing the serum in the form of antigen-antibody complexes, at least when the animal is bled within the first two weeks following the last immunizing injection. Evidence for this has been obtained in vivo and in vitro (3). Thus, antilymphocyte injections may contain not only antibodies but the antigens used to prepare the serum as well, and active immunization to these antigens can result in the human or animal recipient. In the heart-allografted rat, pre-transplantation injection of immune complexes prepared from alloantisera prolongs graft survival. The filtrate from these complexes, presumably containing antigens, also prolongs graft survival when administered in conjunction with nonspecific immunosuppresive drugs after transplantation. These results, however, do not allow determination of whether the effect is active or passive, or perhaps both, nor what is the action of the complexes themselves.