The effect of deep sea water on the fibrinolytic properties of human vascular endothelial cells was investigated. There was no difference in the growth ratio between human umbilical vein endothelial cells (HUVECs) cultured with growth medium (RPMI-1640 containing 20% fetal calf serum) prepared with Hawaii deep sea water (HDSW medium) and those with medium prepared with normal distilled water (control medium). The secretion of type 1 plasminogen activator inhibitor (PAI-1) from HUVECs was significantly reduced by about twofold. However, the levels of PAI-1 mRNA in HUVECs cultured with HDSW medium did not change when compared with those cultured with control medium. Though HDSW medium also reduced the secretion of tissue-type plasminogen activator, the suppressive effect was more prominent for PAI-1. Thus, the balance of fibrinolytic activity was turned toward anti-thrombotic in HUVECs. This was evidenced by the lysis of 125I-fibrin clot in the presence of plasminogen. That is, HUVECs cultured with HDSW medium degraded 125I-fibrin more efficiently than HUVECs with control medium. Such enhanced clot lysis was maintained as long as HDSW medium was present. The accelerated effect of HDSW medium on clot lysis disappeared after the exchange of HDSW medium to control medium. These findings suggest that HDSW may specifically and predominantly affect the process of molecular transfer of PAI-1 after its transcription, resulting in an enhanced fibrinolytic activity of HUVECs. Since HDSW reduces the thrombotic potential of cultured HUVECs, it is speculated that the materials contained in HDSW may prevent the incidence of thrombotic disorders.