Murine polyomavirus contains two related minor coat proteins, VP2 and VP3, in addition to the major coat protein, VP1. The sequence of VP3 is identical to that of the carboxy-terminal two-thirds of VP2. VP2 may serve a role in uncoating of the virus, and both minor coat proteins may be important for viral assembly. In this study, we show that VP3 and a series of deletion mutants of VP3 can be expressed in Escherichia coli as fusion proteins to glutathione S-transferase and partially solubilized with a mild detergent. Using an in vitro binding assay, we demonstrate that a 42-amino-acid fragment near the carboxy terminus of VP3 (residues 140 to 181) is sufficient for binding to purified VP1 pentamers. This binding interaction is rapid, saturable, and specific for the common carboxy terminus of VP2 and VP3. The VP1-VP3 complex can be coimmunoprecipitated with an antibody specific to VP1, and a purified VP3 fragment can selectively extract VP1 from a crude cell lysate. The stoichiometry of the binding reaction suggests that each VP1 pentamer in the virus binds either one VP2 or one VP3, with the VP1-VP2/3 complex stabilized by hydrophobic interactions. These results, taken together with studies from other laboratories on the expression of polyomavirus capsid proteins in mouse and insect cells (S. E. Delos, L. Montross, R. B. Moreland, and R. L. Garcea, Virology, 194:393-398, 1993; J. Forstova, N. Krauzewicz, S. Wallace, A. J. Street, S. M. Dilworth, S. Beard, and B. E. Griffin, J. Virol. 67:1405-1413, 1993), support the idea that a VP1-VP2/3 complex forms in the cytoplasm and, after translocation into the nucleus, acts as the unit for viral assembly.