In ducks two forms of low molecular weight Ig's +), 7.8S and 5.7S, have been described. To compare their antigenicity with Fab and Fc fragments of chicken IgY a double-antibody RIA technique was used in which the binding of 125I-chicken IgY or 125I-chicken IgY (Fc) to rabbit and carp anti-chicken IgY (Fab) and anti-chicken IgY (Fc) antibodies, respectively, was inhibited by duck 7.8S and 5.7S Ig's. Chicken IgY and duck 7.8S Ig ( IgY -like protein) are highly cross-reactive with respect to their Fab as well as Fc part determinants. On the other hand, the duck 5.7S Ig shows nearly identical determinants to the Fab fragments of 7.8S Ig, but a Fc part is lacking. Therefore, we conclude that the 5.7S Ig molecule in ducks doesn't represent a separate Ig class. It consists of the same principal type of heavy chains as the 7.8S Ig, but the H chain of the 5.7S Ig lacks very likely the last two homologous constant regions.