Murine myeloma cells (MOPC-31B) were established in long term culture and retained the capacity to produce and secrete IgG myeloma proteins. The mechanism of synthesis of myeloma protein was analyzed at the cellular and subcellular levels, and the following results were obtained. Light and heavy chains are separately synthesized on different polysomes, 200 S and 280 to 300 S with 12 S and 22 S messenger RNA, respectively. Synthesized heavy chains form H-L complexes with free light chains, which are maintained as a large pool in the cytoplasm, then H2-L2 complexes are formed and secreted after the attachment of carbohydrate in considerable amount.