In the study of the heavy chain from four IgM immunoglobulins no α-amino-terminal group was detected by fluorodinitrobenzene, cyanate, or dimethylaminonaphthalenesulfonyl chloride. The N-terminal residue appears to be a pyrrolidonecarboxylic acid in all four cases. The N-terminal peptide from each of the four has been isolated and a partial sequence determination carried out. The sequences extending over five to nine residues reveal a structural homology among these μ chains and suggest homology with other immunoglobulin chains. © 1968, American Chemical Society. All rights reserved.