Alpha-D-Mannosidase (EC 3.2.1.24.) was purified to homogeneity from adult rat brain. The enzyme, of apparent molecular weight 397,000, appears to be formed of subunits of molecular weight 120,000 made of two protomers (62,000) bound by disulfide bridges. Isoelectric focusing gives two bands, of pI 5.40 and 5.15. Both isoenzymes seem to have the same pH curve (a small peak of activity at pH 4.5 and a maximum of activity around pH 6.0). These two isoenzymes are immunologically related.