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In this work the purification and the complete primary structure of κ-casein from equine milk are reported for the first time. Mares' milk casein was separated by RP-HPLC into four fractions. Complete primary sequence was obtained by sequence analysis of the protein in the fastest eluting peak isolated by chromatography. This sequence was 95% identical to that reported for the C-terminal portion of the zebras' κ-casein and showed high similarity with κ-caseins from sources other than Equidae, confirming that this protein was indeed κ-casein in equine milk. The presence of post-translational modifications in equine κ-casein was investigated by mass spectroscopy, after enzymic dephosphorylation. Two main components were found, the smaller component being more abundant. Equine κ-casein was recognized by a lectin specific for one of the glucosidic bonds in the saccharide moiety of bovine κ-casein. Sequence comparison with prevision studies showed that the distribution of charged and hydrophobic regions in equine κ-casein was similar, but not identical, to that found in the bovine protein; these regions are associated with the role of κ-casein in the formation and stabilization of the micellar structure of casein in milk.