Rabbits were immunized with highly purified desialylated human serum cholinesterase and orosomucoid. In immunoelectrophoresis and immunodiffusion the resulting antibodies had precipitation properties which were identical to those of antibodies against the native proteins. Spurring between native and asialo-proteins was not seen. A linear correlation was observed between the rocket height obtained with the antiserum raised against the asialo-protein and the rocket height with a normal antiserum. A monospecific antiserum against cholinesterase was obtained without absorption. The use of desialylated glycoproteins as immunogens for the production of monospecific antisera is discussed.