Recombinant plasminogen-activator-inhibitor type 1 (PAI-1) purified in an active form from Escherichia coli and eucaryotic cells was found to contain a mixture of three functionally distinct forms: an active form that forms complexes with plasminogen activators (PAs), an inactive (latent) form that remains intact after incubation with PAs, and a substrate-like form which is easily cleaved by PAs. Since active PAI-1 purified from bacteria (rpPAI-1) contains only trace amounts of the inactive latent and the substrate-like forms, this material was used to study the effect of sodium dodecyl sulphate (SDS) on the structure and function of active PAI-1. After treatment with 0.01% SDS, active rpPAI-1 was converted to an inactive form that did not form complexes with PAs, but exhibited characteristics similar to those of latent PAI-1. After treatment with 0.1% SDS, PAI-1 lost its inhibitory activity and was cleaved as a substrate in the reactive center. Circular dichroism spectral analysis reveals that SDS changed the conformation of PAI-1 dramatically, mainly by increasing its alpha-helical content.