A fragment containing J chain was released from human polymeric myeloma IgA protein by cyanogen bromide cleavage. The identity of the fragment was determined by its electrophoretic mobility and antigenic determinants. After purification by gel filtrations and DEAE Sephadex chromatography, this fraction appeared similar (with respect to its amino acid and carbohydrate compositions and its peptide maps) to the J chain isolated from this IgA protein; the molecular weight was 17,000±100. Upon reduction and alkylation, with subsequent separation of peptides by gel filtration, 3 components were obtained: the largest component (molecular weight 13,400) corresponded to the N terminal segment of J chain and contained a homoserine residue, the second corresponded to the C terminal part of J chain with 13 to 18 amino acid residues, and the third corresponded to the C terminal octapeptide of the α chain. The data indicate that J chain is attached to α chain(s) through the penultimate cysteine residue of the C terminal octapeptide.