This study forms part of a larger research project examining the election process for the Nobel prizes for Physiology or Medicine at the Karolinska Institute in Stockholm, and the role and function of the prizes in early 20th century Swedish and international medicine. The purpose of the study is to clarify the decision-making process which led to the Nobel prize for Paul Ehrlich in 1908, 'for work on immunity'. His award was preceded by the most dramatic conflict within the prize authority concerning any prizewinner prior to World War I, and thus is apt to illuminate both the implicit and explicit criteria and the strategies used in the prize deliberations. Ehrlich's chemical ideas on the immune response were criticized by the physical chemist Svante Arrhenius who recommended the application of his disciplines's methods and principles on immunological problems. This criticisms were brought into the Nobel prize debate by J.E. Johansson, a physiologist who asserted that Ehrlich's research was of little scientific value and therefore not worthy of a prize. Yet the majority of the Institute, led by its chairman, the chemist K.A.H. Mörner, succeeded in awarding Ehrlich. An analysis of the controversy shows it to be primarily based upon (1) a difference of scientific styles between the antagonists, resulting in incongruous definitions of immunology as a research field, and of the proper aims and methods of immunological studies. Other factors influencing the final decision were (2) the Institute's negative reaction to what was considered an intrusion in medical Nobel prize matters by a chemist, (3) Arrhenius' and Johansson's diverging views on what kind of work should be awarded a prize, and (4) Johansson's position as a non-conformist at the Karolinska.