Serum of patients undergoing renal homograft rejection show increased levels of an α2-globulin. This serum, as well as an a.-globulin-enriched fraction of normal serum, alters in vitro lymphocyte activity as indicated by decreased 14C-thymidine uptake. These results suggest that the two a-globulins arc identical and that such a serum protein functions as part of an immunoregulatory system.