In antigen-antibody interactions, the high avidity of antibodies for their specific antigens can be achieved both by exploiting the high affinity of each binding site and by multivalence of antibodies. In this study, we developed artificial antibodies with multiple valency. The Fv fragments of an antibody fused with one and with two domains of the Fc-binding protein A from Staphylococcus aureus (designated Fv-P and Fv-PP, respectively) were expressed in secreted forms in Escherichia coli. Their physical characteristics were examined. The Fv portions of Fv-P and Fv-PP had virtually the same affinity for the antigen as that of the original Fv molecules, and each protein A-derived domain could be bound to IgG. When Fv-P was mixed with IgG, the complex formed was composed of two Fv-P molecules and one IgG molecule. In the case of Fv-PP, a complex composed of three Fv-PP molecules and two IgG molecules was preferentially formed. Analysis of surface plasmon resonance in a BIAcore system indicated multivalency of these complexes for antigens. There was 3.5-fold decrease in the dissociation constant of the complex between Fv-PP and the antigen upon the addition of IgG. The use of these complexes as reagents in enzyme-linked immunosorbent assay and Western blotting gave much stronger signals than Fv, Fv-P, and Fv-PP alone.