SUMMARYA deficiency of glycoprotein neuraminidase (sialidase, acylneuraminyl hydrolase, EC 3.2.1.18) activity was found in fibroblasts from a patient with the clinical symptoms of Morquio disease type A (mucopolysaccharidosis IV A ). Residual neuraminidase activity was about 5 % of the mean normal activity. N-Acetylgalactosamine- 6-sulfate (GalNAc-6-S) sulfatase activity was reduced to less than 1 % of normal with a pH-optimum of 3.0 as expected for the severe form of Morquio disease. In peripheral leucocytes of the patient, however, neuraminidase activity but not GalNAc-6-S sulfatase activity was in the normal range. Mixing experiments excluded the presence of excessive amounts of inhibitors of neuraminidase activity.In the urine of the patient, the content of bound sialic acid was with 0.357 nmol/mg creatinine increased, but amounted for only about 10 % of that of a sialidosis patient.Somatic cell hybridization of the patient's fibroblasts with those from a Morquio A patient did not result in complementation of GalNAc-6-S sulfatase activity. Fusion with sialidosis fibroblasts, on the other hand, yielded a significant increase in neuraminidase activity.Reinvestigation of 10 different Morquio A cell lines revealed two of them to have neuraminidase activities below the normal range, whereas the activity of 6 cell lines was in the low normal range.