Scrapie is thought to be caused by one or more conformations of a proteinacious particle called a prion. The infectious form(s) is referred to as the scrapie form of the prion protein (PrPsc) whereas a benign form, the cellular conformer, is referred to as PrPC. The cellular conformation of the sheep prion protein formed a 1:1 complex with human plasminogen. The complex precipitated at 0 degrees C (Ksp = 17* 10(-12) M2). This precipitation reaction was sensitive to both temperature and pressure. When subjected to hydrostatic pressure the precipitate dissolved. At 25 degrees C the complex was soluble with a dissociation constant of about 10(-7) M as determined by isothermal titration calorimetry. Absorption, fluorescence and circular dichroism spectroscopy showed that neither protein, in the complex, underwent a detectable structural change so long as proteolytic inhibitors were present. In the absence of proteolytic inhibitors, plasminogen slowly cleaved the prion protein.