Osteopontin (OPN) is an acidic phosphorylated glycoprotein found in many tissues and physiological fluids. Bovine OPN is a mature protein comprising 262 amino acids with a calculated molecular weight of 29 kDa. However, SDS-PAGE analysis reveals that the protein isolated from milk migrates to a molecular mass of 60 kDa (Sørensen & Petersen, 1993; Bayless et al. 1997). Bovine milk OPN is phosphorylated at 27 serine residues and one threonine residue (Sorensen et al. 1995); three O-glycosylated threonines were also identified, but no asparagine residues were glycosylated in spite of the presence of three putative N-glycosylation sites. In contrast, eight phosphates are recognized in bovine bone OPN (Salih et al. 1996), and 12 phosphoserines and one phosphothreonine are proposed in addition to five O-linked oligosaccharides and at most one N-linked oligosaccharide in the case of rat bone OPN (Prince et al. 1987). Thus, the possibility of tissue or species-specific differences in post-translational modification has been discussed.