Quantitative assessment of the interaction of cell surface receptors with their ligands is mainly based on measurements using radiolabeled or fluorescent ligands. The investigation of the dynamics of the ligand-receptor interaction can easily be followed by binding of fluorescent ligands or monoclonal antibodies to surface receptors and quantitation of cell-associated fluorescence by laser-based flow cytofluorometry. The receptor for the plasminogen activator urokinase (uPAR) on normal cells and on tumor cells is a prominent example to study the dynamics of ligand-receptor interaction. We describe quantitative flow cytofluorometric techniques, applying the urokinase ligands FITC-pro-uPA or FITC-HMW-uPA and the monoclonal antibody (mAB) #3936, which allow to determine uPAR antigen on the surface or inside of a variety of normal blood cells and of tumor cells.