Antiserum to the inhibitor of β-glucuronidase isolated from porcine sublingual gland was prepared in rabbits. Double immunodiffusion with the inhibitor produced a single precipitin line. However, neutralization of the inhibitor was produced by the antiserum and also by normal serum. Anti-β-glucuronidase inhibitor isolated from human serum, by fractionation with (NH 4) 2SO 4 followed by DEAE-cellulose, Sephadex G-200 and Sepharose 4B chromatography, was identified as α 2-macroglobulin by using ultracentrifuge analysis and immunoelectrophoresis. The mechanism of interaction of β-glucuronidase inhibitor with α 2-macroglobulin was also studied.