Peptides and proteins have been widely used to characterize the specificity of peptidases, but frequently the substrates chosen have been physiologically irrelevant. One of the most popular substrates has been the oxidized insulin B-chain, because this is a peptide without tertiary structure, and cleavage depends solely on the preference of the peptidase (13) . (The terms 'specificity', 'selectivity' and 'preference' are used interchangeably here.) However, peptidase preference is exactly that: a preference only. Researchers often find that after prolonged exposure to a peptidase other bonds are degraded, albeit slowly, once none of the preferred bonds remain. If the peptidase preparation is not pure, then there is the danger that some of the observed cleavages are due to contaminating peptidases.