Because of its intrinsic lability, wild-type plasminogen activator inhibitor 1 (PAI-1) cannot be crystallized in its active conformation. Therefore. a stable variant of PAT-I was used to retain the active conformation during crystallization. Four different crystallization conditions were evaluated in detail and two major types of crystals were detected. Whereas solutions consisting of either (i) cacodylate and sodium acetate, (ii) lithium sulfate and polyethylene glycol 4K, or (iii) imidazole, sodium chloride and sodium potassium phosphate: buffer revealed thin platelet crystals a solution (iv) containing ammonium acetate, citrate and polyethylene glycol JE; appeared to enhance the formation of clustered brush-like crystals. Crystals grown under condition (iii) were found to be suitable for X-ray data collection and consequent structural investigation. Data collection was 79.8% complete with a maximum resolution of 2.92 Angstrom. Importantly, PAI-1 retained its functional properties under all conditions.