High-mass MALDI-TOF mass spectrometry (MS) is a novel analytical approach to study large biomolecules and their interactions. It is a powerful alternative method to gel electrophoresis (GE) and size exclusion chromatography (SEC) for obtaining information on the molecular weights of macromolecules and for determining protein complexes. The precision of mass measurements (mass accuracy), high sensitivity, speed of the analysis, and tolerance toward sample heterogeneity are the major features of this MS-based approach. Remarkably, MS provides direct stoichiometric information of macromolecular protein complexes, when noncovalent interactions are stabilized during desorption/ionization by use of chemical cross-linking reagents. In this study, high-mass MALDI-TOF MS was applied to characterize the multimeric state of the human plasma protein haptoglobin (Hp), which is in the mass range of 150-300 kDa. Also, higher order structures of hemoglobin-based oxygen carriers (HBOCs) and their interactions with human haptoglobin were analyzed. These investigations are of clinical importance and contribute to the overall understanding of specific toxicity and clearance of HBOCs.