Studies on the immunological cross-reactivity of the Igs+ from the bile of chicken and turkey (order Galliformes) as well as duck and goose (order Anseriformes) to Igs with antigenic known properties carried out by a series of RIA. Various class-specific anti-Ig sera produced in rabbits, guinea-pigs and mice were used. The following results were obtained: (1) A high extent of antigenic relationship between the chicken and turkey biliary Igs was demonstrated by a radiobinding system consisting of 125I-turkey biliary Ig and class-specific rabbit anti-turkey biliary Ig antibodies. The biliary Igs of duck and goose did not inhibit this antigen binding system. (2) The duck and goose biliary Igs were able to inhibit the binding of 125I-carp IgM to Fc-fragment-specific rabbit anti-carp IgM antibodies and of 125I-human IgM to Fc-fragment-specific guinea-pig anti-human IgM antibodies, respectively. (3) None of the biliary Igs show cross-reactive properties common with human and porcine IgA. That was detected by inhibition of the radio-binding system from 125I-human IgA and alpha chain-specific mouse anti-human IgA antibodies. Therefore, the conclusion was made that in anseriform birds IgM-like Igs are the secretory Igs, while in galiform birds biliary Igs with special antigenic properties occur.