J chain has been detected both immunochemically and by disc electrophoresis in human colostral and salivary IgA, in five polymeric myeloma IgA, and in five macroglobulinemic IgM preparations. Serum 7S IgA, IgG, and myeloma IgD are devoid of J chain. Urinary protein from two patients with IgA myeloma of polymeric type had high levels of Bence-Jones protein but no evidence of J chain. For J chain preparation by ion-exchange chromatography, sulfitolysis was preferable to reduction and alkylation because of a better separation of J chain from other subunits. The proportion of one J chain per 11S secretory IgA and per pentameric IgM molecules was determined. It is argued that J chain is not derived from α chain. © 1972.