Analyzing the effect of ligands on protein-protein interactions is important to better understand the cellular processes. In vitro characterization of these modulations remains challenging because of the drawbacks associated with the analysis of noncovalent interactions. To facilitate the analysis, stabilization of the protein complex by chemical cross-linking followed by High-Mass MALDI mass spectrometry is a recently developed method offering several advantages: No need for immobilization or special tags, the analysis is possible directly on wild-type protein complexes, no need for buffer exchange, large applicability range for any type of protein complex from 0 to 1,500 kDa. Using this method, we analyzed the effect of the inhibitors Nutlin-3a and Nutlin-3b on the protein complex MDM2-p53. Using this fast and sensitive method, the IC(50) values of these inhibitors have been determined.