FROM an evolutionary point of view the earliest immunoglobulins to appear are widely considered to be members of the IgM class, primarily because of their presence in many lower phylogenetic forms1. The IgG class is believed to be of more recent origin. In spite of their biological importance, comparatively little is known of the evolutionary history of IgA molecules as alpha chain amino acid sequence determinations have been limited to small portions of the hinge region2,3, certain cystine peptides4 and the carboxyterminal octapeptide5. Most authors have assumed that IgA diverged from the other classes more recently than did IgG because IgA was difficult to identify in birds, and has not been found in reptiles, amphibians or lower vertebrates. We report here amino acid sequence data on a human alpha chain which imply a close evolutionary relationship between alpha and mu chains.