A double antibody radioimmunoassay for determination of plasminogen activator inhibitor (PA-inhibitor) in plasma samples has been developed. The reliability of the method as assessed by determining the specificity, the accuracy, the detectability and the variability seems sufficient for use in clinical practice. The correlation between PA-inhibitor antigen as measured with this method and PA-inhibitor activity as measured with a spectrophotometric assay in 111 patients with thrombotic diseases was very good (r = 0.89). As calculated from the regression line or from the mean activity and mean antigen values a specific activity of about 800,000-900,000 arb U/mg was obtained for PA-inhibitor in these samples. In 15 healthy individuals a similar figure was obtained. The results suggest that PA-inhibitor in most plasma samples is fully active or close to fully active. PA-inhibitor activity and PA-inhibitor antigen have also been measured after venous occlusion. The data suggest that small amounts of PA-inhibitor is released on venous occlusion, but at the same time an inactivation takes place, most likely due to the formation of enzymatically inactive complex with simultaneously released t-PA.