The effect of trans-5-prostaglandin E2 (trans-PGE2) on fibrinolysis was examined in vitro using synthetic chromogenic substrate S-2251. trans-PGE2 was found to enhance plasminogen (PLG) activation mediated by tissue-type plasminogen activator (tPA). The enhancing effect was dependent on the concentration of trans-PGE2. cis-PGE2 and the other PGs (PGE1 and PGI2) did not show such an effect as trans-PGE2, despite to the fact that their structures are similar to that of trans-PGE2. trans-Configuration around the double bond at the 5-position seems to be important in the enhancement of the fibrinolytic activity.