The technology of humanization of rodent antibodies has opened the way for a broad range of therapeutic antibodies with very low immunogenicity, which are, therefore, suitable for repeated dosing. Such intact antibodies have extended serum half-lives and biodistribution profiles very similar to human antibodies. For some applications, however, the ideal therapeutic should have reduced serum half-life and altered biodistribution patterns typical of antibody fragments, such as Fab or single chain Fv. Bispecific antibody fragments offer exciting additional therapeutic possibilities, but their successful manufacture and purification on a large scale require the development of new methods. Antibody fragments often assemble in Escherichia coli as monovalent fragments with reduced affinities. We describe the spontaneous assembly of bivalent antibody fragments in E. coli and methods of purification that yield either bivalent or monovalent molecules as required.