The serine protease urokinase (uPA, urokinase-type plasminogen activator) is over-expressed in certain tumors and is considered to be the strongest single indicator of poor prognosis in patients with metastatic breast cancer. In this article, we describe the isolation and affinity maturation of a fully human recombinant antibody (termed DS2), specific to the human uPA and capable of inhibiting its enzymatic activity with an IC(50) value in the low nanomolar range. The novel antibody cross-reacts with murine uPA. It was expressed both as scFv fragment and in IgG format, allowing a systematic comparative immunofluorescence (IF) analysis of the uPA expression patterns in a large panel of human und murine tumors and of normal human tissues. Although uPA was strongly expressed in virtually all tumor specimens tested, it exhibited only a weak expression in certain normal tissues (mainly in the colon, lung, spleen and bone marrow). IgG(DS2) was not able to inhibit cancer growth in immunocompromised mice bearing subcutaneous human MDA-MB-231 or DoHH-2 tumors. However, an ex vivo IF analysis confirmed the ability of the DS2 antibody to preferentially localize at the tumor site compared with normal organs. Collectively, these data suggest that uPA blocking antibodies may not be indicated for cancer growth inhibition strategies, but may serve as valuable tools for the implementation of pharmacodelivery strategies against a variety of different tumors.