Both the human and the porcine urokinase-type plasminogen activator (u-PA) genes have been isolated and their entire nucleotide sequences determined. Two distinct inhibitors specific for plasminogen activators have been purified and characterized. One, type 1 plasminogen activator inhibitor has been found in cultures of normal and neoplastic cells, placental tissue, plasma, serum, and blood platelets. The other type, plasminogen activator inhibitor type 2, has been found in placental tissue, leucocytes, and cultured monocyte-macrophages. u-PA was also identified in Sertoli cells in the seminiferous tubules of rat testes, correlated in time and place with release of spermatocytes. A variety of findings relate u-PA produced by macrophages and polymorphonuclear leucocytes to inflammation. A role of u-PA has been proposed in angiogenesis and of PA of non-determined type in nuclear destruction in maturing keratinocytes and in inactivation of the receptor for epidermal growth factor during down-regulation of this receptor in cultured cells.