Plasminogen activator is a protease which catalyses the conversion of the inactive plasminogen to the active plasmin. Most transformed cell lines and solid tumors produce elevated levels of plasminogen activator compared with nontransformed counterparts. This increased synthesis of plasminogen activator may play a role in tumorigenesis, cancer invasion and metastasis. Measurement of plasminogen activator in tumor extracts and body fluids may provide diagnostic and prognostic information. Finally, since plasminogen activator is an estradiol-inducible enzyme, its measurement in breast carcinomas might be a marker for a functional estrogen receptor.