Human N-acetylgalactosamine-6-sulfate sulfatase (6-sulfatase) activity is measured by using as a substrate a sulfated tetrasaccharide obtained by digesting purified chondroitin-6-sulfate (C-6-S) with testicular hyaluronidase. The amount of inorganic sulfate released is measured turbidimetrically. The enzyme from human kidney has a pH optimum of 4.8; its activity is augmented by low levels of NaCl and inhibited by phosphate and high levels of NaCl. Free glucuronate, acetylgalactosamine, inorganic sulfate, polymeric C-6-S, or tetrasaccharide obtained from chondroitin-4-sulfate do not affect the enzyme activity. The method may be used for the diagnosis of Morquio disease since extracts of Morquio fibroblasts are devoid of 6-sulfatase activity.