An activator protein which stimulates the degradation of globotriaosylceramide by human hepatic alpha-galactosidase (alpha-D-galactoside galactohydrolase, EC 3.2.1.22) was isolated from human liver and purified some 1300-fold. The purified activator was heat stable up to 95 degrees C, its molecular weight was estimated at 20,000 by gel filtration. Ampholyte displacement chromatography resolved the activating factor into two fractions with isoelectric points at pH 4.6 and pH 4.8, respectively, with otherwise identical properties. The protein did not stimulate the hydrolysis of the water-soluble 4-methylumbelliferyl-alpha-galactoside.