Summary Lysosomal α-D-mannosidase of monkey brain existed in two forms. One form of mannosidase was bound to the Ricinus communis agglutininl20 (RCA1)-Sepharose and could be specifically eluted with lactose. The other form did not bind to the RCAl-Sepharose. Both forms of mannosidase could bind to a similar extent to the immobilized brain lysosomal receptor protein. Both the forms were purified to apparent homogeneity. Neutral sugar analysis by GLC showed the presence of glucose, mannose and galactose in the RCAl-Sepharose bindable mannosidase and glucose and mannose in the non-bindable mannosidase. Several other brain lysosomal hydrolases did not bind to the RCAl-Sepharose. The results suggested the existence of only high mannose oligosaccharides in the RCAl non-bindable mannosidase and both high mannose and complex oligosaccharides in the bindable mannosidase.