The type of proteinase and the nature of the active site of the human adenovirus proteinase are unknown. For these reasons we produced an inhibitor profile of the enzyme. Enzyme activity in disrupted virions was inhibited by several serine-specific as well as cysteine-specific proteinase inhibitors. Of the inhibitors that worked, the most useful potentially in illuminating the nature of the active site was bovine pancreatic trypsin inhibitor (BPTI), and for this reason we extensively characterized the interaction with BPTI. In disrupted virions, the enzyme is irreversibly inhibited by BPTI with a Ki of 35 nM and a ki of 6.2 x 10(-4) s(-1). One reason enzyme activity is inhibited is that BPTI, a basic protein, precipitates the viral DNA, a cofactor of enzyme activity. In vitro with purified components, BPTI acts as a competitive inhibitor (Ki 2 microM) of the recombinant proteinase complexed with its 11-amino-acid cofactor pVIc. The recombinant endoproteinase is beat labile whereas its 11-amino-acid cofactor is heat stable. We estimate there are about 50 molecules of proteinase per virus particle.