The nucleocapsid-dUTPase protein of Mason-Pfizer monkey virus is a truly bifunctional fusion enzyme. The exact role of this fusion protein in the viral life cycle is unclear. To explore its function, we started to identify interacting protein partners of the enzyme in vitro. Three viral proteins, integrase, capsid and nucleocapsid, were found to be capable of physical interaction with NC-dUTPase. Integrase protein is an important component within the preintegration complex; therefore the present results also suggest that NC-dUTPase might be associated with this complex.