The Hurler syndrome (alpha-L-iduronidase deficiency disease) is a severe lysosomal storage disorder that is potentially amenable to enzyme-replacement therapy. Availability of a canine model of the disease and a sufficient supply of corrective enzyme have permitted a therapeutic trial lasting 3 mo. Recombinant human alpha-L-iduronidase, purified to apparent homogeneity from secretions of a stably transfected Chinese hamster ovary cell line, was administered i.v. to homozygous affected animals in doses of approximately 1 mg. The enzyme rapidly disappeared from the circulation in a biphasic manner, with t1/2 of 0.9 and 19 min, respectively, and was taken up primarily by the liver. Biopsy of the liver before and after a very short trial (seven doses administered over 12 days) showed remarkable resolution of lysosomal storage in both hepatocytes and Kupffer cells. After weekly administration of enzyme to three affected animals over a period of 3 mo, the level of enzyme was about normal in liver and spleen, lower but significant in kidney and lung, and barely detectable (0-5% of normal) in brain, heart valves, myocardium, cartilage, and cornea. Light and electron microscopic examination of numerous tissues showed normalization of lysosomal storage in liver, spleen, and kidney glomeruli, but there was no improvement in brain, heart valves, or cornea. Even though the treated dogs developed complement-activating antibodies against alpha-L-iduronidase, clinical symptoms could be prevented by slow infusion of enzyme and premedication.