Conditioned medium from cultured bovine aortic endothelial cells contains an inactive plasminogen activator inhibitor (PAI). This latent PAI can be "activated" with denaturants. For example, less than 0.01 units/microliter of PAI activity was detected in untreated conditioned medium, but medium treated with sodium dodecyl sulfate (1.7 mM), guanidine HCl (4 M), urea (12 M) or KSCN (6 M) contained 0.9, 1.9, 0.8, and 0.5 units/microliter, respectively. This effect was dose-dependent with respect to the particular reagent used, and the same concentration of reagent which induced PAI activity also stimulated the ability of a component in conditioned medium to form sodium dodecyl sulfate-stable complexes with exogenously added plasminogen activators. Neither activity was stimulated by extensive dialysis or by treatment with NaCl (5 M), Na2SO4 (2.8 M), or dicetyl phosphate (0.1%). Analysis of treated and untreated conditioned medium by gel filtration revealed that the latent and active PAIs migrated with apparent Mr values of 30,000 and 50,000, respectively. Thus, "activation" is associated with an increase in the apparent Mr of the molecule. These observations suggest that activation does not result from the removal of either a small dialyzable component from the medium, or of a large Mr component that is bound to the latent PAI. Other possible mechanisms of activation are discussed. We recently isolated an active PAI from bovine endothelial cells (van Mourik, J.A., Lawrence, D.A., and Loskutoff, D.J. (1984) J. Biol. Chem. 259, 14914-14921). Monospecific antiserum to this active PAI selectivity immunoprecipitated the latent PAI from conditioned medium. These results indicate that the two PAIs are immunologically related and suggest that the latent form is converted into the active form by the sodium dodecyl sulfate present during the purification.