In 2003 some letters, written from 1905-1915 by the father to the chemotherapy, the Nobellaurter Paul Ehrlich (1854-1915) and his family written to Dr. Thorvald Madsen (1870-1957), Director of the State Serum Institute from 1909 to 1940 was found. In these letters the personal and scientific relations between the two scientists is described on the background of the letters found. The article is written with the intention to celebrate the 150-birthday of Paul Ehrlich and to document the close relations between Paul Ehrlich and the founders of the State Serum Institute in Copenhagen: Carl Julius Salomonsen (1847-1924) and Thorvald Madsen.