The titers of hemagglutinins to human erythrocytes were investigated using sera and eggs from hens immunized by a single intraperitoneal injection of crude blood group substance from OSe saliva mixed with Freund's complete adjuvant. Elevation of the hemagglutinin titer in egg yolk followed the increase of serum hemagglutinin with 1- to 2-week time-lag. The maximum hemagglutinin titer of egg yolk was 1/2 to 1/8 of the serum hemagglutinin titer. The total hemagglutinin quantity in egg yolk exceeded the serum quantities of the antibody in sacrificed hens. Egg-white transiently contained a very low titer of hemagglutinin. The yolk hemagglutinin scarcely migrated at all to the white after storage of the egg for 3 months at 5 degrees C. These results indicate that egg yolk from hens immunized with blood group substances is an appropriate source of blood group hemagglutinins.