α1-Acid glycoprotein (orosomucoid) was purified from the human and murine blood sera using phenol deproteinization. As opposed to the murine protein, the human orosomucoid bound the fluorescent dye ethidium bromide but lost this ability after treatment with β-mercaptoethanol, which breaks disulfide bonds. Disulfide bonds between the Cys23and Cys165residues of the human orosomucoid and between the Cys91and Cys184residues of the murine orosomucoid were identified.