Rabbit anti-Beijing duck apo A-I and apo B antisera were prepared by immunizing New Zealand rabbits with duck apo A-I and LDL, respectively. Rocket immunoelectrophoresis and radial immunodiffusion were used to determine the serum levels of apo A-I and apo B of 55 normal Beijing ducks and 8 cholesterol-fed Beijing ducks. Normal ducks contained about 145 mg apo A-I and 73 mg apo B per 100 ml serum; cholesterol-fed ducks contained about 167 mg apo A-I and 75 mg apo B per 100 ml serum.