A murine monoclonal antibody (MAb) to the human insulin receptor (HIR) has been engineered for use as a brain drug delivery system for transport across the human blood–brain barrier (BBB). The HIRMAb was humanized by complementarity determining region (CDR) grafting on the framework regions (FR) of the human B43 IgG heavy chain and the human REI kappa light chain. A problem encountered in the humanization process was the poor secretion of the CDR-grafted HIRMAb by myeloma cells. This problem was solved by the production of human/mouse hybrids of the engineered heavy chain variable region (VH), which led to the replacement of five amino acids in the FR3 of the VH with original murine amino acids. No replacement of FR amino acids in the light chain variable region (VL) was required. The affinity of the humanized HIRMAb for the HIR was decreased 27% relative to the murine HIRMAb. The humanized HIRMAb avidly bound to the HIR of isolated human brain capillaries, which are used as an in vitro model system of the human BBB. The HIRMAb cross reacts with the HIR of Old World primates such as the Rhesus monkey. The humanized HIRMAb was radiolabeled with 125-iodine, and injected intravenously into an adult, anesthetized Rhesus monkey. Brain scanning showed the humanized HIRMAb was rapidly transported into all parts of the primate brain after intravenous administration. The humanized HIRMAb may be used as a brain drug and gene delivery system for the targeting of large molecule therapeutics across the BBB in humans. Biotechnol. Bioeng. 2007;96: 381–391. © 2006 Wiley Periodicals, Inc.