Natural erythrocyte agglutinins in the serum of the Australian freshwater catfish, Tandanus tandanus (Mitchell). I. Examination of the specificities of the agglutinins with emphasis on the ABH agglutinins.

Normal serum samples from the Australian freshwater catfish, (Mitchell), were shown to have haemolytic and haemagglutinating activity for a variety of animal erythrocytes. Specific agglutinins were present for the erythrocytes of most animals tested but some cross-reactivity of agglutinins was demonstrated. Evidence was obtained that the greater part of the human erythrocyte agglutinating activity was directed toward antigens of the ABH system. Whereas some sera clearly showed anti-H or anti-A and B agglutination patterns, others agglutinated equally erythrocytes of different groups. ABO erythrocytes were agglutinated to higher titres than were erythrocytes of the `Bombay' phenotype, salivas from secretors inhibited the agglutination of erythrocytes carrying appropriate ABH antigens, whereas salivas from non-secretors did not and absorptions of sera often revealed ABH specific agglutinins. While some sera agglutinated equally ABO erythrocytes because they contained a mixture of agglutinins with different ABH specificities, other serum samples containing one predominating species of agglutinin caused equal agglutination of O, A, AB and B erythrocytes. The specificity of this latter type of agglutinin is discussed.