Letter from Kai O. Pedersen to Michael Heidelberger

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, during his second visit, antibodies in horse antipneumococcal sera. In experiments completed by Pedersen after Heidelberger's departure and discussed by Pedersen in this letter, the two found that these antibodies were of high molecular weight, another indication that they were, in fact, proteins.