The synthesis, assembly, and secretion of the three major subclasses of mouse IgG has been examined in 14 myeloma tumors and two cultured cell lines as well as in the cells from the popliteal lymph nodes of immunized mice. The total amount of IgG synthesized was between 15 and 43% of the cytoplasmic proteins made during a 15 min period. H2 and H2L were the major precursors of IgG2a and IgG1 but, in all of the tumors, HL was also an intermediate. In contrast, HL was a major precursor of IgG2b. Most of the noncovalent and covalent assembly of IgG occurred after release of the newly synthesized H and L chains from the polyribosomes and assembly was not completed until 10 min or more after the synthesis of the polypeptide chains.

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