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WE have already investigated the production of immunoglobulin G (IgG) by mouse plasma cell tumour 5563 and have shown that light chains are released autonomously from polyribosomes into a pool of free light chains. The time course of incorporation of radioactive amino-acids into free light chain and the myeloma protein indicated that the pool of free light chains is small and reaches maximum radioactivity after about 10 min of incubation. It behaves as a pool with rapid turnover from which light chains appear to be incorporated into G myeloma protein molecules1,2. These findings led us to propose the possibility that the pool of free light chains controls the release of heavy chains from the polyribosomes.

THE plasma cell tumour designated X5563 originated in a 22½-month-old female C3H/He mouse in the Cancer Research Genetics Laboratory, Berkeley, California. It was successfully transplanted by Dr. H. I. Pilgrim, and mice bearing the second transplant generation were sent to the National Cancer Institute, Bethesda, Maryland, where it was routinely transplanted every 60 days in C3H/He mice for several transplant generations1,2. Mice inoculated with the tumour were sent to us by Dr. M. Potter, of the National Cancer Institute. The tumour is now in its third transplant generation in our hands.