The primary structure of the J chain from a human Waldenströms IgM protein has been determined using a combination of automated and conventional Edman degradative procedures. Eighty-five percent of the sequence was established with peptides isolated from tryptic digests of carboxyamidomethylated and citraconylated J chain, many of which were sequenced completely. Alignment of the tryptic fragments was achieved with peptides generated by chymotrypsin and limited acid hydrolyses. The J chain consists of 129 amino acids and a single oligosaccharide structure linked to asparagine at position 43 of the sequence. The molecular weight, including 7.5% carbohydrate by weight, is 16 422. The location and arrangement of three half-cystines could be deduced from previous studies, whereas the pairing of the remaining five disulfide bonds still needs to be clarified. © 1977, American Chemical Society. All rights reserved.