In 1966 Jerne moved back to Europe and took up the position of Professor of Experimental Therapy at the Johann Wolfgang Goethe University in Frankfurt. From 1966 to 1969 he was the Director of the Paul-Ehrlich-Institut, also in Frankfurt. In 1969 Jerne again switched jobs, this time to Basel in Switzerland, where he was the Director of the Basel Institute for Immunology until his retirement in 1980. During the 1970s and 1980s, Jerne was a pioneer in the development of immune network theory.