The doors for the natural sciences development were opened with the establishment of the modern University of Zagreb (1874) and its science department (1876). This development also initiated the establishment of the study of pharmacy (1882) at the Faculty of Philosophy. Julije Domac, the university professor of pharmacognosy, founded the first independent Institute of Pharmacognosy (1896). In this way he introduced the pharmacognosy as an independent science at the University of Zagreb. His assistant, Antun Vrgoč (1881 – 1949), came after him, initially as an associative professor and head of the Institute of Pharmacognosy (1924) and later as the full professor of pharmacognosy (1925) and the first Dean of the Faculty of Pharmacy of the University of Zagreb (1942 – 1945). On the initiative of professor Domac, Vrgoč continued his scientific education at the Pharmacy Institute of the University of Bern (1912 – 1914), where followed classes by the prominent pharmacognosy professor Alexander Tschirch. Consequently he returned to Zagreb and introduced his enriched professional experience to the Croatian scientific community. Mutually exchanged letters between two professors serve as a proof of their professional cooperation and their friendship (1920 – 1939). For the first time in this graduation thesis the original letters of Tschirch (37 letters) as well as letters of his family (four letters), sent to Vrgoč, including the translations of those letters are presented. The analyses of Vrgoč and Tschirch's correspondence give insight in the long-lasting expert and scientific cooperation between the two university professors. Preserved and translated Tschirch's letters show honest and friendly relationship between two professors, based on mutual understanding, appreciation and respect.