Thyroglobulin has a characteristic absorption curve in the ultraviolet region of the spectrum. The two absorption maxima of this curve are in the neighborhood of 2800B and of 3200 A. The former is due chiefly to the absorption peaks of tyrosin and tryptophane. The latter is caused almost entirely by the absorption of thyroxin and diiodotyrosin. Methods developed by Caspersson (to be published) make it possible to measure directly and quantitatively with the aid of a photoelectric cell the absorption of ultraviolet light in a small, definitely circumscribed area of about 0.5 sq. p of a thin section of an organ such as the thyroid. Even smaller areas can be studied by his photographic methods ( '36). These procedures permitted us to take advantage of the specific ultraviolet light -absorbing proper ties of thyr oglobulin to determine quantitatively the concentration of total protein in the colloid, and of thyroxin + diiodotyrosin in both colloid and cells.
Attempts were made to localize the iodine-containing substances in the gland cells. The tyrosin and tryptophane content of the colloid is regarded as a measure of its protein concentration. This assumption is justified by the analyses of Cavett, who showed that the amino acid composition of thyroglobulin (except for its iodine-containing moieties) remains 'Thanks are due to the Rockefeller Foundation and the Th6rBse and John a Fellow of the John 8irnon Guggenheim Memorial Foundation. Anderason Memorial Fund for the optical instruments used.