The complete amino acid sequence of the 432-residue heavy (alpha) chain of mouse myeloma MOPC 511 has been determined. The variable region of the alpha chain of IgA 511, a phosphocholine-binding protein, is highly homologous to that of the other phosphocholine-binding immunoglobulins. Comparison of the 511 alpha chain constant region with that of other mouse and human heavy chains shows that sequence divain. The CH3 domain disulfide bridge of the 511 alpha chain, for example, consists of only 28 amino acid residues compared to 60 residues for other chains and domains. Sequence divergences are alsos apparent at the CH2/CH3 domain boundary, an area where a number of frameshift mutations have occurred. One mutant, mouse IgA 47A, lacks the entire CH3 domain. Comparison of the 511 alppha chain with the 47A alpha chain reveals two noncconservative amino acid changes at the COOH terminus of the 47A chain, Ser-Gln for VAl-Thr in the 511 chain. These changes and the deletion of the CH3 domain can be explained by a single genetic event--namely, a frameshift mutation followed by premature chain termination. The remainder of the 47A constant region, including the hinge region, is identical to the 511 alpha chain, except for two conservative changes in the CH1 domain: serine-126 and theonine-197 in the 511 alpha chain are both replaced by alanine in the 47A chain.