Lysosomal beta-mannosidase was purified almost 10,000-fold from human placenta. The final preparation showed several protein bands on polyacrylamide gel electrophoresis. Its molecular mass was estimated to be 110 kDa, the optimal pH was 4.5, the Km was 0.56 mM, and the isoelectric point was 4.7. The enzyme was found to bind completely to Con A-Sepharose, and the pI was not changed after neuraminidase treatment. These results indicate that the purified enzyme represents a lysosomal form which contains high mannose type oligosaccharide chains and only a few sialic acids, if any.