Structural differences between the heavy chain of membrane-bound IgM (mu m) and the heavy chain of secreted IgM (mu s) were investigated. The primary translation products of the mu-chain, free of posttranslational modifications, were synthesized in a wheat-germ cell-free system, programmed with messenger RNA derived from human lymphoblastoid cell lines positive for both membrane-bound and secreted IgM. Encoded in this sytem were two mu-chains, which shared N-terminal signal peptides and which differed both in molecular weight and in C-terminal amino acid sequence. In vivo pulse labeling of cells confirmed that, as intermediates in the rough endoplasmic reticulum, these two forms expressed the same idiotype and maintained their difference in molecular weight and in C-terminal sequence. By correlation with pulse-chase kinetics and with immunofluorescence, one form of mu-chain represents mu m, and the other, mu s. Because the molecular weight difference between the two is manifest at the level of their primary translation products, these studies demonstrate that mu m is distinguished from mu s by a difference in primary structure, at least in part at the C-terminus.