The cysteine protease encoded by adenovirus type 2 contains eight cysteines, some of which are involved in catalysis and enzyme activation. Here we investigated the effects of oxidation, mercapto-ethanol, dithiothreitol, diamide and protein disulphide isomerase on wild-type and mutant enzymes. Three isoforms of the enzyme were detected in infected cells and a fourth in preparations of purified recombinant enzyme. The latter isoform was absent in preparations of enzyme mutated at any of the three conserved cysteines, C-104, C-122 and C-126. Enzyme activity could be stimulated by agents other than the authentic activating peptide (pVIc), such as cysteamine, though less efficiently. Diamide at low concentrations stimulated the activity of the ts1 enzyme, but inhibited both ts1 and wild-type enzyme at higher concentrations. Protein disulphide isomerase failed to restore enzyme activity to the oxidized isoform. The present studies in combination with previous results using mutants appeared to rule out amino acids C-67, C-122, C-126 and C-127, leaving the two remaining semi-conserved C-17 and C-40 and the conserved C-104 as potential candidates for binding peptide pVIc.