A glycoprotein was isolated from human plasma which partially inhibited C3 carrying erythrocytes from binding to complement receptor cells (CR+C). Based on its physicochemical characteristics and its antigenicity this glycoprotein was identified as alpha 1-antitrypsin (alpha 1-AT). The activity of alpha 1-AT towards C3 and its fragments was unaffected by heating but it was destroyed by periodic acid. The isolated carbohydrate moiety of alpha 1-AT showed the same effect as the intact molecule. Using F(ab)2 of IgG-anti-alpha 1-AT could be demonstrated on Raji cells and human erythrocytes. Treatment of these CR+C with IgG-anti-alpha 1-AT resulted in a blockade of their C3 receptor activity. The results suggest, that alpha 1-AT interacts through its carbohydrate portion with C3 and its fragments and functions as a complement receptor molecule.