Mucopolysaccharidosis type I (MPS I, Hurler and Scheie syndromes) is an autosomal recessive lysosomal storage disorder that results from a deficiency of the hydrolase alpha-L-iduronidase (IDUA) which is involved in the lysosomal degradation of both heparan sulphate (HS) and dermatan sulphate (DS). Patients with MPS I store and excrete large amounts of partially degraded HS and DS. In order to evaluate enzyme replacement therapy for MPS I patients we have expressed human IDUA cDNA in Chinese Hamster Ovary (CHO)-K1 cells utilizing a plasmid vector that places the cDNA under the transcriptional control of the human polypeptide-chain-elongation factor I alpha gene promoter. A clonal cell-line that secreted recombinant IDUA in a precursor form at approximately 2.2 micrograms/10(6) cells per day was identified. This enzyme was shown to be endocytosed into cultured MPS I fibroblasts via mannose-6-phosphate receptors and to correct the storage phenotype of these cells by enabling the lysosomal-digestion of accumulated sulphated glycosaminoglycans. The recombinant IDUA had on SDS/PAGE a molecular mass of 85 kDa and was processed to 74 kDa and smaller forms following its uptake by fibroblasts. Milligram quantities of the recombinant IDUA were immunopurified and the enzyme was shown to have pH optimum and kinetic parameters differing from those of the mature enzyme purified from human liver. The specific activity of the recombinant enzyme was shown to increase on dilution and on incubation with reducing agents. This was in contrast to the mature IDUA form (74 kDa) which did not have its activity stimulated by reducing agents or dilution.