We investigated the effect of plasma and serum from 10 subjects with the lupus anticoagulant and thrombosis and 9 normal subjects on the secretion of tissue-type plasminogen activator (t-PA) and its rapid inhibitor (type 1 plasminogen activator inhibitor, or PAI-1) by cultured human endothelial cells. Confluent monolayers of human umbilical vein endothelial cells were incubated for 48 hours with plasma or serum diluted ten-fold in serum-free endothelial cell growth medium, and the secretion of t-PA and PAI-1 measured by enzyme-linked immunosorbent assay. No consistent differences in mean t-PA and PAI-1 release were found between cells exposed to normal plasma or serum and plasma or serum from subjects with the lupus anticoagulant and thrombosis. No plasma or serum sample produced consistent inhibition of t-PA release or stimulation of PAI-1 release (defined as t-PA levels less than the mean minus two standard deviations for normal subjects, and PAI-1 levels greater than the mean plus two standard deviations for normal subjects, respectively). These findings do not support a role for altered endothelial fibrinolytic activity in the pathogenesis of thrombosis in subjects with the lupus anticoagulant, and are consistent with previous observations that these subjects have normal fibrinolytic activity in vivo.