The activation peptides of procarboxypeptidases of the A1 type act in a post-activation mechanism of control, limiting the expression of activity immediately after the limited proteolysis has occurred. This effect is due to structural complementarity between the enzymatic moiety and the released fragments, and is further enhanced by the occurrence of procarboxypeptidase A1 in oligomeric complexes of different type depending on the species studied. In those complexes, the activation segment has an important role in the stabilisation of the quaternary structure. The relationship between oligomeric structure and activation is reviewed here for two of the most extensively studied systems.