AbstractHaptoglobin (Hp) is one of the acute phase proteins (APP) in the blood of vertebrates that is involved in immune responses. Hp concentrations are found to vary under conditions of, for example, infection, trauma or cancer. These variations and the changes in its constitution are frequently used to assess the health status of mammals. In this work, Hp from the blood plasma of diseased and healthy harbor seals was isolated and structurally characterized. The process developed for the isolation of Hp is based on glycoprotein enrichment from crude plasma samples by means of ConA lectin affinity separation. Structural features of the protein backbone and the N-glycans were determined using MALDI-TOF/TOF-MS. De novo sequencing of seal Hp revealed an α-chain composed of 84 amino acids and a β-chain comprising 245 amino acids. Comparison with Hp of the phylogenically related dog and human Hp 1−1 reveals the conserved and variable regions. All cysteine residues responsible for disulfide bonds and one glycosylation site have identical positions in the primary structures. Altogether, four possible glycosylation sites were identified. The glycoprofile is dominated by disialylated biantennary complex-type glycans.