A 6-sulfated tetrasaccharide obtained by digesting chondroitin-6-sulfate with testicular hyaluronidase was used as a substrate for the determination of N-acetylgalactosamine-6-sulfate sulfatase activity. The activity was not detected in liver obtained from the elder sister with clinically classic Morquio syndrome and 4.7% of the control in liver from the younger sister with the same disease.