Summary A homogeneous glycoprotein, orosomucoid, isolated from normal human serum was found to be weakly antigenic in chickens. The antibody-antigen system appeared to consist of a single antigen and its specific antibody. A procedure for determining immunochemically the orosomucoid content of serum has been described. The results show a reasonably good correlation of orosomucoid with the levels of the seromucoid fraction (mucoprotein) of normal and pathological serum.