Mucopolysaccharidosis IIIB (MPS IIIB) is a lysosomal storage disorder caused by mutations in NAGLU, the gene encoding alpha-N-acetylglucosaminidase. The disease is characterized by profound mental retardation and eventual neurodegeneration, but relatively mild somatic manifestations. There is no available therapy. We have used a mouse knockout model of the disease to test therapy by genetically modified bone marrow. Bone marrow from Naglu -/- male mice was transduced with human NAGLU cDNA in an MND-MFG vector, and transplanted into 6- to 8-week-old lethally irradiated female -/- mice. Sham-treated mice received bone marrow transduced with eGFP cDNA in an MND vector. alpha-N-Acetylglucosaminidase activity in plasma and leukocytes, measured 3 and 6 months after transplantation, varied from marginal to nearly 30 times wild-type. A low level of alpha-N-acetylglucosaminidase activity, as little as provided by transplantation of unmodified Naglu +/+ bone marrow, could normalize biochemical defects (glycosaminoglycan storage and beta-hexosaminidase elevation) in liver and spleen, but a very high level was required for an effect on kidney. Effects on the brain were best seen by examination of cellular morphology using light and electron microcopy. Mice that expressed very high levels of alpha-N-acetylglucosaminidase in blood had an increased number of normal-appearing neurons in the cortex and other parts of the brain, while microglia with engorged lysosomes had almost completely disappeared. Immunohistochemistry showed a marked decrease of staining for subunit c of mitochondrial ATP synthase and for Lamp1, markers of neuronal and microglial pathology, respectively, as well as a decrease in staining for glial fibrillary acid protein, a marker of activated astrocytes. These results show that genetically modified cells of hematopoietic origin can reduce the pathologic manifestations of MPS IIIB in the Naglu -/- mouse brain.