Single-chain Fv proteins are known to aggregate and form multimeric species. We report here that these molecules represent a new class of molecular assembly, which we have termed multivalent Fvs. Each binding site in a multivalent Fv comprises the variable light-chain (VL) domain from a single-chain Fv, and the variable heavy-chain (VH) domain from a second single-chain Fv. Each single-chain Fv in a multivalent Fv is part of two binding sites. We have characterized the multivalent forms of the 4-4-20, CC49 and B6.2 sFvs. The degree of multivalent Fv formation is linker-dependent. Multivalent Fvs cannot form in the absence of an intact linker. Multivalent Fvs can be stabilized by their antigen. The conversion between different forms of the multivalent Fvs can be catalyzed by disassociating agents such as 0.5 M guanidine hydrochloride with 20% ethanol. Multivalent Fvs have significantly different stabilities depending on the specific variable domains from which they are constructed. Two models have been proposed for the structure of a multivalent Fv. We have tested each model by attempting to produce a heterodimer from the anti-fluorescein 4-4-20 and anti-tumor CC49 variable regions. We successfully produced a 4-4-20/CC49 heterodimer that comprises two mixed sFvs. The first mixed sFv is composed of the 4-4-20 VL domain, a 12 residue linker and the CC49 Vh domain. The second mixed sFv is composed of a CC49 VL domain, a 12 residue linker and the 4-4-20 VH domain. The 4-4-20/CC49 heterodimer bound both fluorescein and the tumor-associated glycoprotein-72 antigen. These results support a VH/VL 'rearrangement' model in which each variable domain of a multivalent Fv binding site comes from a different polypeptide chain.