The MALDI-MES provides a rapid, sensitive and reproducible alternative approach to existing analytical techniques for the detection of enzymatic activities that does not require a chromophore or radiolabeling. An improved method is presented, by which enzymes with defined substrate specificities can be detected with a MALDI mass spectrometer in complex protein fractions. In order to demonstrate the utility of the new method, in this study we describe the use of MALDI-MES to detect proteolytic activities in a protein extract from porcine renal tissue, which contained several thousand proteins as visualized by 2D electrophoresis. The analytical procedure is based on covalent immobilization of proteins to beads. By immobilizing proteins, autolytic and proteolytic degradation is prevented and the removal of those molecules from the protein fraction is achieved, which otherwise would interfere with the mass spectrometric detection of the enzymatic reaction products. The enzymatic activity is determined by incubating the immobilized proteins with a reaction-specific probe, followed by the analysis of the reaction mixture with the MALDI-MS after defined incubation times. The presence of the target enzyme is validated by locating a signal, which fits the molecular mass of the expected reaction product in the mass spectrum. To demonstrate how to detect proteolytic activities in this system, the reactions catalyzed by endopeptidase, angiotensin-converting enzyme, kallikrein, renin, and urotensin-converting enzyme were monitored. The experiments showed that the MALDI-MES method is sufficient according the quantification to investigate the effects of inhibitors. This is demonstrated using a specific renin inhibitor to inhibit an angiotensin-I generating enzyme activity in a renal protein extract.