Four different H-chains from IgM proteins (μchains) have been studied from the standpoint of their gross molecular characteristics. These chains were shown to be homogeneous from the standpoint of sedimentation in the ultracentrifuge and migration in polyacrylamide gel disc electrophoresis. From the pattern of elution by gel filtration the molecular weight for all four calculated to be approximately 75,000. Each of the polypeptide chains had approximately 10% hexose, a C-terminal tyrosine residue, and a blocked N-terminal residue. Tryptic peptide maps suggested that the primary structure of these molecules have a significant proportion of their sequence in common, but a length of variable sequence that is limited to a perhaps relatively small part of the polypeptide chain. © 1969.