Letter from Michael Heidelberger to Kai O. Pedersen

Twice, in 1934 and 1936, Heidelberger received a John Simon Guggenheim Memorial Fellowship for a stay of six weeks at the University of Uppsala in Sweden, where he used the first ultracentrifuge, constructed by Theodor Svedberg and Kai O. Pedersen, to determine the molecular weight of various proteins, including antibodies in horse antipneumococcal sera. During his first visit, Heidelberger brought purified thyroglobulin, a protein secreted by the thyroid gland. In experiments completed by Pedersen after Heidelberger's departure and discussed by Heidelberger in this letter, the two found to their surprise that thyroglobulin was of high molecular weight.