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Total polysomes were obtained by homogenization of sheep thyroid glands in Tris-C1-buffered sucrose, pH 7.5, treatment with 2 ' %; Triton X-100 and centrifugation of the magnesium-treated 27 000 x g supernatant through 1 M sucrose. Purification of thyroglobulin-specific polysomes was performed by indirect immunoprecipitation with rabbit antibodies to sheep thyroglobulin and donkey antibodies to rabbit immunoglobulin G. Thyroglobulin polysomes represented 30 yo of total polysomes. Synthesis of thyroglobulin peptides in a reticulocyte cell-free system, programmed with total RNA from immunoprecipitated polysomes, was 3 times higher than when total RNA from unfractionated polysomes was used. The size of the poly(A)-containing thyroglobulin mRNA was estimated by the capacity of RNA fractions obtained by sucrose density gradient centrifugation of total or immunopurified polysomal RNA to hybridize with 3H-labeled poly(U) and to stimulate thyroglobulin peptide synthesis in a reticulocyte lysate. 65 of the thyroglobulin messenger activity was located in a 33 -36-S RNA peak and 35 in zones corresponding to 30-S and 26-S RNA species.The thyroglobulin-specific immunoprecipitable peptides synthesized in the reticulocyte lysate from the 33 -36-S RNA were shown by polyacrylamide gel electrophoresis in denaturing conditions to contain a major component co-migrating with the constituent peptide chain of sheep thyroglobulin ( M , 330000). Other components migrated with M , of about 210000 and 120000.

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