We have investigated whether the saponin astragaloside IV (AS-IV), a 3-0-β-D-xylopyranosyl-6-O- β -D-glucopyranosylcycloastragenol, purified from the Chinese herb drug Astragalus membranaceus, which is used in traditional Chinese medicine to treat cardiovascular diseases, might affect the fibrinolytic potential of cultured human umbilical vein endothelial cells (HUVECs). When HUVECs were conditioned with AS-IV, a dose (0.01–100 µg AS-IV/ ml)- and time-dependent decrease in plasminogen activator inhibitor type 1 (PAI-1) and an increase in tissue-type plasminogen activator (t-PA) synthesis were observed, which were significant from 1 µg AS-IV/ml and from 12 h of incubation with 100 µg AS-IV/ml. PAI-1 antigen decreased from 641 ± 86 to 318 ± 18 ng/l0<sup>5</sup> cells/24 h, whereas t-PA antigen increased from 4.1 ± 0.3 to 9.7 ± 0.4 ng/l0<sup>5</sup> cells/24 h after addition of 100 µg AS-IV/ml. PAI-1 activity decreased to 30% of control level, whereas t-PA activity and t-PA-PAI-1 complexes reached a maximum stimulation of 3- and 5-fold over control levels, respectively, in the conditioned media of HUVECs treated with 100 µg AS-IV/ml for 24 h. PAI-1-specific mRNA expression decreased to 55% (2.2 kb) and 72% (3.2 kb), 66% (2.2 kb) and 88% (3.2 kb), and 19% (2.2 kb) and 41% (3.2 kb) of control values after incubation for 6, 12 and 18 h, respectively, whereas t-PA-specific mRNA increased 2-, 2.5- and 1.4-fold in HUVECs treated with 100 µg/ml AS-IV for 6, 12, and 18 h, respectively. In conclusion our data give evidence that in fact AS-IV can increase the fibrinolytic potential of cultured HUVECs not only by upregulating the expression of t-PA as NG-Rl does, but also by downregulating the expression of PAI-1.