Here we report the crystal structure of a stablilized plasminogen activator inhibitor-1 variant (PAI-1-N150H-K154T-Q301P-Q319L-M3541 (PAI-1-stab)) that shows a cleavage within the reactive centre loop. The new structure is of superior quality compared to the previously determined structure of the cleaved PAI-1-A335P mutant. We present a detailed comparison of the two structures and also compare them with the structure of the active PAI-1-stab. The structural data give important insights into the working mechanism of PAI-1 and also explain the role of various stabilizing mutations. (C) 2008 Elsevier Inc. All rights reserved.