Four types of immunoglobulin (Ig) have been identified in ducks: IgM, a secretory Ig resembling IgM, a 7.8S IgG, and a 5.7S IgG. Structurally and antigenically the 5.7S IgG resembles an F(ab')2 fragment of the 7.8S IgG. When ducks mount serum antibody responses, the sequence of Ig involvement is IgM --> 7.8S IgG --> 5.7S IgG. Although serum Ig levels increase, and antigen-binding Igs can be demonstrated, sera from repeatedly immunized ducks commonly lack secondary antibody activities such as agglutination, precipitation, complement fixation and tissue sensitization. These deficiencies are most likely attributable to the absence of functionally important components of the predominant Ig (5.7S IgG) and/or a possibly unusual steric structure of duck Igs. A related issue concerns production of the two antigenically related IgGs: what are the cellular and molecular events involved, and how are they controlled? Evidence from current molecular genetic studies has confirmed the similarity of the VH, CH1 and CH2 domains of the 7.8S and 5.7S IgGs and shown, by virtue of the existence of separate mature messages for the heavy (H) chains of these molecules, that they are biosynthesized independently. Models for the possibilities that the two H chains are products of one gene or of two genes are presented. Cloning and sequencing the duck H chain gene locus, which is in progress, is providing data supporting the one gene hypothesis. The results obtained from cDNA sequencing also confirm that the duck IgGs are unusual in terms of the anatomy of the hinge region and of the number and location of intra- and inter-chain disulphide bonds, observations which will be of importance for understanding structure/function relationships of these unusual and interesting molecules.