Thyroglobulins (Tg) 27S and 19S and their hormonal content were determined in normal (N = 6), paranodular (N = 4) and hypofunctioning nodular (N = 8) human thyroid tissues. The proportion of Tg 27S and the hormonal content of Tg 19S were significantly decreased in cold nodules. In normal tissues, the molar ratio of T4 in Tg 27S (2.8) was almost twice that found in Tg 19S (1.7), while for T3 the molar ratio in Tg 27S (0.06) was lower than in Tg 19S (0.09). These results indicate that Tg 27S is predominantly formed from well iodinated T4-containing Tg 19S molecules.