Three out of four cloned mouse plasmacytoma cell lines examined synthesized and secreted polymeric IgA with covalently bound J chain, thus establishing that J is synthesized by the plasma cell. A fourth cell line synthesized and secreted only monomer (7S) IgA which contained no J chain. In the immunoglobulin secreted by the polymer-secreting lines, J was not bound to monomer and was added to polymer at or very near the time of polymerization. These results are consistent with the suggestion that polymer assembly is mediated by J chain.