Elucidation of the reactive site loop (RSL) structure of serpins is essential for understanding their inhibitory mechanism. Maintenance of the RSL structure is likely to depend on its interactions with a dominant unit of secondary structure known as the A-sheet. We investigated these interactions by subjecting alpha 1-proteinase inhibitor to limited proteolysis using several enzymes. The P1-P10 region of the RSL was extremely sensitive to proteolysis, indicating that residues P3'-P13 are exposed in the virgin inhibitor. Following cleavage eight or nine residues upstream from the reactive site, the protein noncovalently polymerized, sometimes forming circles. Polymerization resulted from insertion of the P1-P8 or P1-P9 region of one molecule into the A-sheet of an adjacent proteolytically modified molecule. The site of cleavage within the RSL had a distinct effect on the conformational stability of the protein, such that stability increased as more amino acids insert into the A-sheet. We conclude that the A-sheet of virgin alpha 1-proteinase inhibitor resembles that of ovalbumin, except that it contains a bulge where two or three RSL residues are inserted. Insertion of seven or eight RSL residues, allowed by proteolytic cleavage of the RSL, causes expansion of the sheet. It is likely that the RSL of alpha 1-proteinase inhibitor and several serpins exhibits significantly more mobility than is common among other protein inhibitors of serine proteinases.