After the end of the war Hirszfeld and his wife returned to Warsaw, where he established a serum institute modeled after the Ehrlich Institute for Experimental Therapy in Frankfurt. He soon became the deputy director and scientific head of the State Hygiene Institute in Warsaw and, in 1924, professor there. In 1931 he was named full professor at the University of Warsaw and served on many international boards. After the occupation of Poland by the German Army Hirszfeld was dismissed as a "non-Aryan" from the Hygiene Institute but, through the protection of friends, managed to do further scientific work at home until February 1941; it was, however, almost impossible for him to publish.