Abstract: For selectively and reversibly adsorbing tPA, pro-tPA or mixtures thereof, from an aqueous medium, for example, derived from a melanoma cell culture, the aqueous medium is intimately contacted with an affinity reagent comprising an immobilized Kunitz-type inhibitor, substantially as occurs in and is extractable from seeds of an Erythrina species, e.g., Erythrina latissima, having Erythrina species, the following combination of characteristics:(a) it is an inhibitor of the Kunitz-type;(b) it inhibits trypsin;(c) it inhibits plasmin;(d) it has no effect on urokinase;(e) it inhibits tPA; and(f) in its immobilized form, it is a selected adsorbent for tPA and its precursor pro-tPA, to thereby cause the tPA, pro-tPA or mixtures thereof to become selectively adsorbed on the affinity reagent.