The relationships among the thyroid iodoproteins, their biosynthesis in vitro in thyroid slices and the histological structure of a multinodular, sporadic goitre was examined after strumectomy from a euthyroid 51-year-old woman. Using sedimentation methods 27S, thyroglobulin (TG), 12S and 3--8S proteins were found. Besides these, 4 to 5 other proteins with molecular weights between 165,000 and 36,000 daltons were detected by polyacrylamide gel electrophoresis. The concentration of soluble proteins was very low (3 mg/100 mg wet tissue), particularly TG (0.69 MG/100 MG). The sedimentation constant (18.3S) of goitrous TG was lower than mature 19S-TG and it was poorly iodinated (0.06% w/w). 27S iodoprotein was present in the goitre extract. Thyroid slices of goitre tissue incorporated 14C-leucine into proteins and synthesized TG and its subunits. Newly synthesized proteins were rapidly released from the microsomes. Pathohistologically, struma colloido-microfolliculare with dystrophic-proliferative changes was noted. In the large follicles which were filled with colloid, dystrophic alterations in thyrocytes and desquamation of the follicular epithelium were observed. In conclusion, it is suggested that the presence of poorly iodinated, immature TG in nontoxic multinodular, sporadic goitre, in the absence of iodine deficiency, probably results in disturbances in the utilization of TG from the follicular lumen. In the follicular lumen under these conditions it is possible that 27S protein is formed from pre-existing poorly iodinated TG. These findings suggest that inability to resorb colloid may be a cause of nodule formation.