The exploitation of biological macromolecules, such as nucleic acids, for the fabrication of advanced materials is a promising area of research. Although a greater variety of structural and functional uses can be envisioned for protein-based materials, systematic approaches for their construction have yet to emerge. Consistent with theoretical models of polymer macrocyclization, we have demonstrated that, in the presence of dimeric methotrexate (bisMTX), wild-type Escherichia coli dihydrofolate reductase ( DHFR) molecules tethered together by a flexible peptide linker (ecDHFR(2))are capable of spontaneously forming highly stable cyclic structures with diameters ranging from 8 to 20 nm. The nanoring size is dependent on the length and composition of the peptide linker, on the affinity and conformational state of the dimerizer, and on induced protein-protein interactions. Delineation of these and other rules for the control of protein oligomer assembly by chemical induction provides an avenue to the future design of protein-based materials and nanostructures.