Introduction of the complementary determining regions (CDRs) from a murine antibody to a human monoclonal antibody is an important technique (humanization) in the development of human immunotherapeutics. We have humanized murine monoclonal antibody HCMV37 which binds to the gB envelope glycoprotein of human cytomegalovirus. Simple transfer of the murine HCMV37 CDRs into heavy- and light-chain framework regions (FRs) based on human NEW and REI, respectively, together with human IgGI and K constant regions, abolished antigen binding because of a suboptimal heavy chain. Replacement of human VH amino acids Leu70, Val71 and Arg94 with murine residues Thr70, Arg71 and Asn94 was insufficient to improve affinity. However, significant restoration of binding was obtained by substitution of human VH amino acids Thr28, Phe29, Ser30 with murine residues Ser28, Ile29, Thr30, in conjunction with the position 94 change. Residue 71, often regarded as critical for antigen binding, was not a major factor.