Most conventional analytical tools for the assessment of protein-protein interactions yield information on the bulk sample. By employing the efficient separation of intact proteins, affinity capillary electrophoresis (ACE) can measure the interaction of components of heterogeneous proteins with a target protein. In this work, the hyphenation of ACE with mass spectrometry (MS) is presented as a novel, highly selective tool for the assessment of protein-protein interactions. The binding of the protease inhibitor aprotinin to trypsinogen was used as protein-protein affinity model. A trypsinogen sample comprising several modifications was analyzed using a background electrolyte of 25 mM ammonium acetate (pH 8.0) containing increasing concentrations of aprotinin (0-300 μM). A capillary coating of polybrene-dextran sulfate-polybrene (PB-DS-PB) was employed to prevent adsorption of the proteins to the capillary wall. The trypsinogen variants were separated and could be assigned based on detected molecular masses and relative migration. In presence of aprotinin, both free and aprotinin-bound trypsinogen were detected revealing a 1:1 binding stoichiometry. For most trypsinogen variants, shifts in electrophoretic mobility were observed upon raising the aprotinin concentration, allowing determination of their dissociation constants (Kd's). The interacting trypsinogen variants showed similar affinity toward aprotinin (Kd's of 3-9 μM), which were not significantly different from the values obtained with ACE-UV and were in agreement with an earlier reported value. The use of the ratio of obtained MS signal intensities of free and protein-protein complex for the determination of Kd's was also explored. Derived Kd values (20-104 μM) for the binding variants were similar to those obtained with direct-infusion MS, but higher and less precise as compared with values based on mobility shifts. The suitability of the ACE-MS methodology for the affinity profiling of heterogeneous protein samples was evaluated, and components with high, medium, or low affinity toward aprotinin could be successfully discriminated. (Figure Presented).