The serpins SCCA1 and SCCA2 are highly expressed in the epithelium of the conducting airways, a common site of infection by group C adenoviruses, such as human adenovirus type 2 (Ad2). Based on the common location we examined a possible interaction between them. In vitro experiments with recombinant proteins showed that SCCA1 inhibited the viral protease in a dose-dependent manner. Both serpins were cleaved in a manner consistent with hydrolysis within their reactive site loop, without the formation of an SDS-resistant complex, as in the case of papain. Infection of SCCA1-expressing cells did not result in the cleavage of SCCA1, nor was the yield of infectious virus affected as compared to SCCA1-negative parental cells. This may be due to differential localization, the serpin being cytoplasmic and viral protease being nuclear. Surprisingly, however, virus infection, which tends to inhibit host protein synthesis, caused a significant increase in SCCA1 expression well into the late phase of infection.