The plasminogen activator exists in the human placenta (PPA) as a reversible complex with its inhibitor (UKI). Plasminogen activating activity appeared resulting from the separation of UKI from PPA-UKI complex through UK-Sepharose or UK-Affi Gel 10 affinity chromatography. This crude PPA was purified through gel filtration on Sephadex G-150 and DEAE-Sephacel column chromatography. The purified PPA was obtained at a rate of 25 micrograms from one placenta, the specific PA activity of which was 21, 071 IU/mg-protein. The molecular weight of PPA was estimated at about 65,000 (unreduced) by SDS-polyacrylamide gel electrophoresis. PPA did not react to the anti-UK serum and UK did not react to the anti-PPA serum. It is revealed that PPA has a smaller influence on fibrinogenolysis than UK does. PPA is a new plasminogen activator in the human placenta, which has different properties from UK biologically and immunologically. It is speculated that in the near future PPA will be one of the tissue activators in thrombolytic therapy for thrombotic diseases including toxemia. We note that the measurement of the plasma PPA level is useful as one of the marker proteins of placental function and malignant tumors.