Distribution of protein and I131 in centrif ugally fractionated thyroid homogenates, and the electrophoretic, solubility and ultracentrifuge characteristics of the soluble fraction were studied in normal and diseased human thyroid glands. In the normal glands, 64-82 percent of the protein and 95 percent of the I131-label appeared in the soluble fraction. Thyroglobulin comprised 70-79 percent of the soluble protein. Normal thyroglobulin had a Svedberg constant (0.5 percent solution in 0.9 percent NaCl) of 18-20, and salted out of phosphate buffer at 1.7-1.9 [image] No protein with greater mobility than thyroglobulin was detected on paper electrophoresis in barbital buffer. In all diseased glands, an increased proportion of light-weight (SW 20=4) protein was observed, and the thyroglobulin usually had greater solubility in phosphate buffer. In the soluble fraction of some colloid goiters, 1 follicular adenoma, 3 congenital goiters and 1 thyroid carcinoma, a protein as yet indistinguishable from albumin was detected. An abnormal iodinated soluble protein was detected in 1 congenital goiter, in which gland thyroglobulin was absent. Colloid goiters were associated with an increased proportion of soluble protein and I131, whereas congenital goiter and carcinoma were associated with an increased proportion of particulate protein and I131. The alterations observed are apparently due to (1) an accumulation of albumin-like protein in colloid goiters, some follicular adenomas and some congenital goiters, and (2) a deficient production of thyroglobulin in some congenital goiters and in thyroid carcinoma. It is suggested that abnormalities in the metabolism of proteins other than thyroglobulin could play a role in the production of nodular colloid goiter.