A series of human cell lines has been examined for fibrinolysis in culture. The sera that are activating for fibrinolysis by human cells are mouse, monkey, human, horse, and bovine. Individual human sera show considerable variation in the ability to activate fibrinolysis. In common with other neoplastic or transformed mammalian and avian cell cultures, human cell lines of neoplastic origin produce substantial amounts of plasminogen activator. Several cultures of nonmalignant origin also produce plasminogen activator, whereas cultures obtained from trypsinized human embryos, or from human embryonic skin do not. The human melanoma plasminogen activators are of two kinds: a major component with a mol wt of 50,000, and a minor species with a mol wt of approximately 60,000. Both are DFP sensitive, serine proteases.

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