The acidic α-D-mannosidase (EC 22.214.171.124) has been purified from human placentae. Milligram quantities of the enzyme were obtained from several placentae, using a step-wise purification procedure which includes Con A-Sepharose treatment, acetone precipitation,heat treatment, DEAE-cellulose column chromatography and preparative disc electrophoresis. A high degree of purity of the purified enzyme was demonstrated by acrylamide gel electrophoresis, isoelectric focusing, and sedimentation equilibrium centrifugation. Immunological homogeneity of the preparation was demonstrated by a single precipitin line between the antiserum and purified, or partially purified enzyme preparation. The amino acid and carbohydrate composition of the enzyme was determined. The enzyme was found to be a glycoprotein containing 13.5% carbohydrate. The molecular weight of the enzyme was estimated as 205,000 ± 18,400.