Enzyme replacement therapy has been used successfully in many lysosomal storage diseases. However, correction of brain storage has been limited by the inability of infused enzyme to cross the blood-brain barrier. The newborn mouse is an exception because recombinant enzyme is delivered to neonatal brain after mannose 6-phosphate receptor-mediated transcytosis. Access to this route is very limited after 2 weeks of age. Recently, several studies showed that multiple infusions of high doses of enzyme partially cleared storage in adult brain. These results raised the question of whether correction of brain storage by repeated high doses of enzyme depends on mannose 6-phosphate receptor-mediated uptake or whether enzyme gains access to brain storage by another route when brain capillaries are exposed to prolonged, high levels of circulating enzyme. To address this question, we used an enzyme whose carbohydrate-dependent receptor-mediated uptake was inactivated by chemical modification. Treatment of human beta-glucuronidase (GUS) with sodium metaperiodate followed by sodium borohydride reduction (PerT-GUS) eliminated uptake by mannose 6-phosphate and mannose receptors in cultured cells and dramatically slowed its plasma clearance from a t(1/2) of <10 min to 18 h. Surprisingly, PerT-GUS infused weekly for 12 weeks was more effective in clearing central nervous system storage than native GUS at the same dose. In fact, PerT-GUS resulted in almost complete reversal of storage in neocortical and hippocampal neurons. This enhanced correction of neuronal storage by long-circulating enzyme, which targets no known receptor, suggests a delivery system across the blood-brain barrier that might be exploited therapeutically.