Carboxyamidomethylated J chain was shown to be an excellent substrate for the enzyme, pyrollidone carboxylyl peptidase, which specifically removed the cyclized amino terminal glutamyl residue. J chain lacking the "blocked" PCA group was subjected to automated Edman degradation and the amino terminal amino acid sequence determined as: PCA-Glu-Asp-Glu-Arg-Ile-Val-Leu-Val-Asp-Asn-Lys-CMCys-Lys-CMCys-Ala-Arg. Previous studies by others have identified a disulfide bridge between the heavy chain of immunoglobulins and a tripeptide identical in composition with the sequence at positions 15-17 in the J chain. These two sets of data locate the linkage of immunoglobulin heavy chain with Cys 15 of the J chain. © 1976.