Amino acid compositions provide a reliable guide to sequence similarities.

the substrate alone, but in the presence of the totally inactive trypsin-inhibitor complex, a re-activation of trypsin and neutral proteinase activity was observed. This can only be accounted for by the replacement of trypsin and neutral proteinase in the enzymeinhibitor complex by enzymically inactive Tos-Phe-CH,CI-chymotrypsin. Chymotrypsin and its precursor chymotrypsinogen have no proteolytic activity on fluorescein-labelled polymeric collagen fibrils that can be assayed by direct fluorimetry of solubilized peptides. Yet chymotrypsinogen can be shown to be capable of reactivating trypsin and neutral proteinase that have previously been completely inactivated by the tumour inhibitor (Fig. 2) . In an exactly similar manner, Trasylol-trypsin (having no proteolytic activity on the substrate alone) releases trypsin and neutral proteinase previously bound in their enzyme-inhibitor complexes. In these two cases, the regain of proteolytic activity is a time-dependent process (Fig. 2) dependent upon the concentration of added competitor. Clearly the enzyme-inhibitor complex can be dissociated by suitable donors of disulphide groups located at a site other than the active centre of these enzymes. These data indicate an allosteric control of certain proteolytic activities mediated by thioldisulphide exchange.