Plasminogen is the zymogen form of plasmin, a broad specificity serine protease whose activity contributes to a variety of normal and pathological conditions, including intravascular thrombolysis and extracellular proteolysis. Plasminogen contains seven structural units or 'domains', each of which confer specific properties on the molecule. The kringle domains possess fibrin-binding functions and, together with the N-terminal peptide, regulate the ability of plasminogen to adopt at least three dissimilar conformations. These conformational forms influence the rate of formation, following activation by plasminogen activators, of the plasmin active site within its C-terminal serine protease domain. Structural and functional analogies are postulated between these plasminogen structures and the conformations of other proteins related by sequence homology.