Mucopolysaccharidosis Type I, Hurler's Syndrome, is a lysosomal storage disorder that affects the brain. The missing enzyme, α-L-iduronidase (IDUA), does not cross the blood-brain barrier (BBB). To enable BBB transport of the enzyme, human IDUA was fused to the carboxyl terminus of the heavy chain of a chimeric monoclonal antibody (MAb) to the human insulin receptor (HIR). The HIRMAb crosses the BBB on the endogenous insulin receptor, and acts as a molecular Trojan horse to ferry into brain the IDUA. Transfection of COS cells resulted in high levels of IDUA enzyme activity both in the medium and in the intracellular space. The size of the fusion heavy chain, as measured with Western blotting and antibodies to either human IDUA or human IgG, was increased about 80 kDa, relative to the size of the heavy chain of the parent HIRMAb. The IDUA enzyme specific activity of the affinity purified HIRMAb-IDUA fusion protein was 363 ± 37 U/µg protein, which is comparable to specific activity of recombinant IDUA. The accumulation of glycosoaminoglycans in Hurler fibroblasts was decreased 70% by treatment with the HIRMAb-IDUA fusion protein. Confocal microscopy showed targeting of the fusion protein to the lysosome. The HIRMAb-IDUA fusion protein bound with high affinity to the HIR, and was rapidly transported into the brain of the adult Rhesus monkey following intravenous administration. The HIRMAb-IDUA fusion protein is a new treatment for Hurler's syndrome, which has been specifically engineered to cross the human BBB. Biotechnol. Bioeng. 2008;99: 475–484. © 2007 Wiley Periodicals, Inc.