Antibody variable domains differ considerably in stability. Single-chain Fv (scFv) fragments derived from natural repertoires frequently lack the high stability needed for therapeutic application, necessitating reengineering not only to humanize their sequence, but also to improve their biophysical properties. The human V(H)3 domain has been identified as having the best biophysical properties among human subtypes. However, complementarity determining region (CDR) grafts from highly divergent V(H) domains to huV(H)3 frequently fail to reach its superior stability. In previous experiments involving a CDR graft from a murine V(H)9 domain of very poor stability to huV(H)3, a hybrid V(H) framework was obtained which combines the lower core residues of muV(H)9 with the surface residues of huV(H)3. It resulted in a scFv with far better biophysical properties than the corresponding grafts to the consensus huV(H)3 framework. To better understand the origin of the superior properties of the hybrid framework, we constructed further hybrids, but now in the context of the consensus CDR-H1 and -H2 of the original human V(H)3 domain. The new hybrids included elements from either murine V(H)9, human V(H)1 or human V(H)5 domains. From guanidinium chloride-induced equilibrium denaturation measurements, kinetic denaturation experiments, measurements of heat-induced aggregation and comparison of soluble expression yield in Escherichia coli, we conclude that the optimal V(H) framework is CDR-dependent. The present work pinpoints structural features responsible for this dependency and helps to explain why the immune system uses more than one framework with different structural subtypes in framework 1 to optimally support widely different CDRs.