A monoclonal or polyclonal antibody directed against urokinase plasminogen activator receptor (u-PAR), or a subsequence, analogue or glycosylation variant thereof. Antibodies are disclosed which react with free u-PAR or with complexes between u-PA and u-PAR and which are capable of 1) catching u-PAR in ELISA, or 2) detecting u-PAR, e.g. in blotting, or 3) in radioimmunoprecipitation assay precipitate purified u-PAR in intact or fragment form, or 4) is useful for immunohistochemical detection of u-PAR, e.g. in immunostaining of cancer cells, such as in tissue sections at the invasive front, or 5) inhibits the binding of pro-u-PA and active u-PA and thereby inhibits cell surface plasminogen activation. Methods are disclosed 1) for detecting or quantifying u-PAR, 2) for targeting a diagnostic to a cell containing a u-PAR on the surface, 3) for preventing or counteracting proteolytic activity in a mammal. Methods for for selecting a substance suitable for inhibiting u-PA/u-PAR interaction, for preventing or counteracting localized proteolytical activity in a mammal, for inhibiting the invasion and/or metastasis comprise the use of the antibodies and of nude mice inoculated with human cancer cells which are known to invade and/or metastasize in mice and having a distinct color, f.x. obtained by means of an enzyme and a chromogenic substrate for the enzyme, the color being different from the cells of the mouse.