STUDIES of the three chief classes of immunoglobulins in the rabbit, IgG, IgA and IgM, have shown that they all possess the basic four chain structure proposed by Porter1 of two light and two heavy chains, and that while the light chains are common to all three classes the heavy chains, γ, α and μ respectively, are class specific, and differ considerably as judged by pep tide mapping2,3. Despite these differences in the heavy chains the allotypic specificities at the a locus, Aa1, Aa2 and Aa3 (ref. 4), are present not only on the γ chain but also on the α and μ-chains5–7. These specificities have been shown to be on the Fd fragment of γ-chain8 and appear to correlate with the N-terminal sequence of that chain9. In the light of these findings it seemed reasonable to expect that similar structures would be present in the N-terminal sequence of both a and μ-chains, and I have attempted to show this by determining the N-terminal sequence of α-chain from rabbits homozygous for the allotypes Aa1 and Aa3.