Plasminogen activator inhibitor-1 (PAI-1) is a member of the serine protease inhibitor (serpin) supergene family and a central regulatory protein in the blood coagulation system. PAI-1 is unique among serpins in exhibiting distinct active and inactive (latent) conformations in vivo. Though the structure of latent PAI-1 was recently solved, the structure of the short-lived, active form of PAI-1 is not known. In order to probe the structural basis for this unique conformational change, a randomly mutated recombinant PAI-1 expression library was constructed in bacteriophage and screened for increased functional stability. Fourteen unique clones were selected, and shown to exhibit functional half-lives (T1/2S) exceeding that of wild-type PAI-1 by up to 72-fold. The most stable variant (T1/2 = 145 h) contained four mutations. Detailed analysis of these four mutations, individually and in combination, demonstrated that the markedly enhanced functional stability of the parent compound mutant required contributions from all four substitutions, with no individual T1/2 exceeding 6.6 h. The functional stability of at least eight of the remaining 13 compound mutants also required interactions between two or more amino acid substitutions, with no single variant increasing the T1/2 by > 10-fold. The nature of the identified mutations implies that the unique instability of the PAI-1 active conformation evolved through global changes in protein packing and suggest a selective advantage for transient inhibitor function.