Tartrate-resistant acid phosphatase was isolated from serum and spleen of patients affected by Gaucher's disease. Electrophoretic and antigenic properties were compared to the enzyme isolated from hairy cells described in a previous study (9). The enzyme isolated from Gaucher serum has electrophoretic and antigenic properties identical to the acid phosphatase band 5b of hairy cells. The major tartrate-resistant acid phosphatase in the Gaucher spleen is band 5a. Bands 5a and 5b have identical protein structure indicated by their identical antigenicity. The removal of carbohydrate from band 5a by sialidase converted band 5a to 5b.