A 101/2-year-old patient with Sanfilippo syndrome type B, after moderate immunosuppression, was infused with 2.2×1010 leukocytes from his normal histocompatible brother. The patient's leukocytes lacked α-N-acetylglucosaminidase activity, while in the donor's cells this activity was in the range for homozygous normal individuals. Cultured patient and donor lymphocytes could also be distinguished on the basis of different chromosome fluorescence staining patterns. Both enzymatic assays and cytogenetic studies indicated that, for the first three weeks after infusion, 20% to 40% of circulating leukocytes had been derived from the donor. After the infusion there was a transient increase in the ratio of small to large glycosaminoglycans in the urine, without change in the clinical status. There was no long-term improvement in the clinical or biochemical manifestations of the disease.