We have isolated from monkey (Macaca radiata) brain lysosomal fraction by phosphomannan-Sepharose chromatography a protein that binds four different lysosomal enzymes, beta-hexosaminidase, beta-glucuronidase, alpha-L-fucosidase and arylsulfatase. The isolated protein which appeared in an aggregated homogeneous form on gel electrophoresis under non-denaturing conditions at both pH 8.3 and pH 5.0 was found to be heterogeneous on SDS-gel electrophoresis with molecular weights less than 67,000. Binding was partly abolished by periodate treatment or by alkaline phosphatase treatment of the lysosomal enzymes. Binding was completely abolished by pronase digestion of the binding protein. Of the different sugars tested for inhibition of binding, mannose-6-phosphate was most effective followed by mannose and N-acetyl glucosamine while glucose and fucose were ineffective.