Iduronate sulphatase (IDS) is responsible for mucopolysaccharidosis type II, a rare recessive X-linked lysosomal storage disease. The aim of this work was to evaluate the functional importance of each N-glycosylation site, and of the cysteine-84 residue. IDS mutant cDNAs, lacking one of the eight potential N-glycosylation sites, were expressed in COS cells. Although each of the potential sites was used, none of the eight glycosylation sites appeared to be essential for lysosomal targeting. Another important sulphatase co- or post-translational modification for generating catalytic activity involves the conversion of a cysteine residue surrounded by a conserved sequence C-X-P-S-R into a 2-amino-3-oxopropionic acid residue [Schmidt, Selmer, Ingendoh and von Figura (1995) Cell 82, 271-278]. This conserved cysteine, located at amino acid position 84 in IDS, was replaced either by an alanine (C84A) or by a threonine (C84T) using site-directed mutagenesis. C84A and C84T mutant cDNAs were expressed either in COS cells or in human lymphoblastoid cells deleted for the IDS gene. C84A had a drastic effect both for IDS processing and for catalytic activity. The C84T mutation produced a small amount of mature forms but also abolished enzyme activity, confirming that the cysteine residue at position 84 is required for IDS activity.