Eight human IgA1 myeloma proteins were analysed by SDS-PAGE. These experiments showed that purified IgA1 proteins comprise both fully S-S bonded and partly S-S bonded molecules. Pepsin digestion of the IgA1 proteins yielded three four-chain and two two-chain fragments. The four-chain fragments are likely to be derived from intact IgA through cleavage of its alpha chains at different sites: between the CH2 and CH3 domains or in the hinge region. The occurrence of F(abc) (ab') fragments, with alpha chains of different lengths, showed that the alpha chains of IgA can be cleaved independently at the hinge region site. The two-chain pepsin fragments must originate from IgA molecules, which lack inter-assay-chain disulphide linkages. The fragments F(abc)2 and Fabc tended to form dimers, probably through non-covalent interactions of their CH2 domains. An immunoblotting method was used to identify Fd-, CH2- and CH3-specific anti-IgA antibodies. The CH2-specific antibodies could be subdivided into antibodies recognizing an isotype present on both four-chain and two-chain molecules or on two-chain molecules only.