Here, Heidelberger and the Swedish physical chemist Kai O. Pedersen discussed the results of their effort to determine the molecular weight of antibodies, which had been made possible by recent advances in purifying antibodies from antibody-antigen precipitates. Measuring antibodies in absolute units of weight would allow testing of hypotheses about their chemical constitution. Using the newly developed method of ultracentrifugation, by which particle size could be evaluated from the rate of sedimentation, Heidelberger and Pedersen found that while the weight of rabbit antibody was probably very similar to that of its principal normal globulin, type I horse pneumococcus antibody weighed anywhere between three and four times that of its principal normal globulin component. The two openly wondered if this discrepancy explained the varying chemical and physiological differences between horse and rabbit antibodies. Pedersen helped develop the first ultracentrifuge, at the University of Uppsala, and this article marked the first occasion on which he was credited for his assistance.