The interaction of alpha1-antitrypsin with trypsin and chymotrypsin has been investigated by protease activity assays, by electrophoretic analysis, by CD and absorption difference spectra, and by gel filtration of reaction mixtures containing excess inhibitor or excess protease. When alpha1-antitrypsin is present in excess, only one stable inhibitor - protease complex is formed. In the presence of excess protease, however, this primary complex is degraded relatively rapidly to one or more secondary complexes. These latter conversions are more pronounced in the case of the antititrypsin-chymotrypsin system. The greater lability of the antitrypsin-chymotrypsin system is evidenced by the relatively rapid release of inactive chymotrypsin from the secondary antitrypsin - chymotrypsin complex. Only minimal amounts of active protease were released from the complexes on the addition of excess protease and one protease could not displace the other from the complex, although competition experiments showed that chymotrypsin reacted more rapidly with the inhibitor than trypsin.