Hens were immunized intraperitoneally with OSe saliva mixed with Freund's complete adjuvant, and the properties of hemagglutinins in sera and egg yolk were compared. Both serum and egg yolk hemagglutinins agglutinated O, A1, B and A1B erythrocytes to a similar degree, and both showed somewhat stronger agglutinating potencies at 5 degrees C than at 25 degrees C or 37 degrees C. The degrees of agglutination were little improved by the addition of 20% BSA or PVP medium solution for potentiating incomplete agglutinin. The antiglobulin-augmentation reaction with specific anti-chicken-Igs revealed that the principal immunoglobulins of the serum hemagglutinins produced during the early and hyperimmune phases of immune response were IgM and IgG, respectively; whereas the presence in egg yolk of both IgM and IgG hemagglutinins was demonstrated throughout the antibody producing periods. The yolk probably contained IgM incomplete hemagglutinins, because addition of an antiserum to chicken-IgM strongly enhanced the yolk hemagglutinin. The secretor status of six saliva specimens, OSe, Ose, ASe, Ase, BSe and Bse, was tested by the agglutinin-inhibition reaction using one of the following reagents: Ulex lectin, chicken antiserum, and egg yolk hemagglutinin. High inhibition titers of OSe saliva were obtained with the serum hemagglutinin (of the early and hyperimmune phases) and the yolk hemagglutinin of the eggs laid in the hyperimmune phase. The test with Ulex lectin showed a moderate grade of inhibition, and the use of yolk hemagglutinin produced in the early stage of immune response resulted in very low inhibition titers. Yolk hemagglutinins produced in the early and hyperimmune stages were more strongly inhibited by ASe and BSe salivas than by other hemagglutinin reagents. These results indicate the availability of blood group agglutinins in the yolks of eggs laid during the hyperimmune stage.