The invention provides a process for producing a purified plasminogen activator substance based on the TPA/pro-TPA couple. Pro-TPA yielding cell cultures are cultivated in a suitable culture medium in the absence of enzymes which catalyse the conversion of pro-TPA into TPA or where such enzymes are present in the presence of a suitable inhibitor adapted to inhibit such enzymes. Accord- : ingly the couple is represented in the harvesting liquid by at least 50 to 100% of pro-TPA and can in fact be virtually 100% pro-TPA, any balance being TPA. The harvesting fluid is treated with an affinity reagent comprising an immobilised Kunitz type inhibitor of the type occurring in seeds of Erythrina latissima and other Erythrina species (DE-3) to selectively adsorb the pro-TPA and any TPA in the harvesting fluid, followed by desorption of pro-TPA and optionally TPA. The pro-TPA so recoverable has 1/10 the plasminogen activating properties of TPA. It is convertable into active TPA enzymatically, e.g. by plasmin or by enzymes present in fibrin clots. The pro-TPA has a higher affinity for fibrin and better selectivity than TPA. It is used for the management and prophylaxis of thrombosis and embolism in dosages of pro-TPA/TPA couple which result in less than 50%, e.g. less than 20% of the plasminogen activating activity resulting from up to 15 mg/24 hrs pure TPA.