An a-mannosidase was purified over 90-fold from jack bean seedlings by succesive chromatography with an overall yield of 6%. Electrophoresis under denaturing conditions gave two subunits with molecular masses of 66,000 (H-subunit) and 44,000 (L-subunit), respectively. With periodic acid-Schiff reagent, only the H-subunit was stained for carbohydrate. Upon N-Glycanase digestion, the molecular mass of the H-subunit was slightly reduced. Binding experiments of the protein toward some lectins suggested that the subunit of the alpha-mannosidase contained not an O-linked, but an N-linked oligosaccharide, which is either a high mannose or complex type, without a nonreducing terminal mannose or a sialic acid residue.