1. The concentration of thyroid albumin was determined in 54 normal and pathological human thyroid tissue specimens obtained at surgery from 34 subjects who had received 125I prior to thyroidectomy. 2. The saline-soluble proteins were fractionated by gel filtration on Sephadex G-200 and the absorbance of the effluent containing albumin and hemoglobin was measured at 280 and 414 nm. The serum albumin content of the blood contaminating the tissue extract was calculated on the basis of hemoglobin determination (A414) and hematocrit values. This "intravascular albumin" was present in all extracts studied but accounted for less than 10% of the total thyroid albumin in normal tissue and even less in pathological specimens. "Extravascular albumin" was taken as the difference between total and "intravascular" albumin. 3. The amount of extravascular albumin in normal tissue (N = 20), 14.9 +/- 1.9 mg/g thyroid, was significantly lower than in adenomas (hot nodules, N = 7, 32.0 +/- 4.1; cold nodules, N = 16, 24.0 +/- 2.7), papillary carcinomas (N = 9, 51.1 +/- 16.7) and Hashimoto's thyroiditis (N = 2, 36.1 and 176.3). 4. In vivo labelling by 125I of the albumin fraction was demonstrated by Sephadex G-200 gel filtration and by cellulose acetate electrophoresis. Although the ratio of concentration of total albumin/thyroglobulin determined by electrophoresis was similar for 4 normal (0.263 +/- 0.046) and 5 pathological tissues (0.268 +/- 0.059), the relative distribution of radioactivity (total albumin/thyroglobulin) was significantly higher for pathological tissue (0.091 +/- 0.007) than for normal tissue (0.052 +/- 0.023), P less than 0.05. 5. The data suggest that most of the extravascular thyroid albumin is found within the follicle, and demonstrate that there is a significant increase in the concentration of extravascular thyroid albumin associated with several thyroid disorders.