Abstract: Minactivin (also known as Plasminogen Activator Inhibitor-2 [PAI-2]), a protein inactivator of urokinase-type plasminogen activator, has been shown to be a natural inactivator of this plasminogen activator which is associated with invasive tumors, and is therefore indicated as a crucial element in the body's normal defense against tumor invasion and metastasis. It may be produced by the cultivation of minactivin-producing cells in vitro, and recovery of the cell culture supernatant. By controlling the culture conditions, the protein minactivin may be produced in a partially purified form which may be used for diagnosis and treatment of tumors. The specification discloses purification of biologically active native minactivin, as well as peptides derived from minactivin and their amino acid sequences.