A common method for the activation of mammalian metalloproteinases is the use of mercurial compounds. Activation of PMNL procollagenase by soluble mercurials takes place as a three-step mechanism with a final intermolecular loss of the PRCGVPD autoinhibitor region. In this study covalently bound mercury in the form of mercurial agarose was chosen to probe activation of PMNL procollagenase. Activation was not achieved, since the final intermolecular cleavage with removal of the PRCGVPD motif could not take place. An intermediate form of the enzyme was bound to the column. Its N-terminal sequence determination proved cleavage of the Asp64-Met65 peptide bond leaving the cysteine of the propeptide domain for covalent attachment to the mercurial agarose. This gives further evidence of a cysteine-switch mechanism involving Cys71.