The conversion of plasminogen to active plasmin is thought to be a crucial step in the process of extracellular matrix degradation associated with metastatic spread. Activation of plasminogen is initiated by urokinase plasminogen activator (uPA). The binding of uPA to the uPA cell surface receptor (uPA-R) accelerates plasmin generation from plasminogen and localizes uPA activity to the cell surface. We investigated the mRNA-expression of uPA-R in 19 different human breast cancer cell lines. In a competitive reverse transcription polymerase chain reaction (cRT-PCR) we simultaneously co-amplified two different RNA templates bearing the same primer recognition sequences, the cell line RNA and a known amount of an in vitro synthesized uPA-R-RNA internal standard. We analyzed the two PCR products differing 50 bp in size by agarose gel electrophoresis and calculated the initial uPA-R-RNA template concentration from the relative intensities of the bands quantified by video densitometry. We grouped the investigated cell lines according to their in vitro invasiveness according to literature. Cell lines with a high potential of invasiveness showed a higher expression of uPA-R compared to those with a low potential of invasiveness (Student's t-test, p 0.04). In addition to that we compared the uPA-R mRNA levels with uPA-R, uPA, and PAI-1 protein levels in culture supernatants and cell lysates. The obtained results in breast cancer cell lines with different invasiveness and in benign epithelial cell lines revealed the complex cooperation of the urokinase type proteolytic pathway. uPA, uPA-R, and PAI-1 are to be considered as a diagnostic tool rather than assaying a particular molecule alone. Our findings support the hypothesis that the urokinase proteolytic pathway plays a central role in the acquisition of an invasive phenotype and favors its potential use as a prognostic marker in patients with breast cancer.