With his work on the purification of penicillin, he obtained an MSc degree in 1946. He went for further training under (later Nobel Prize winners) Hugo Theorell at the Karolinska Institutet in Stockholm, and Carl and Gerti Cori at the Washington University in St. Louis. He joined the faculty of medicine at Leuven in 1947. In 1960 he was invited to the Rockfeller Institute (now Rockefeller University). With mutual arrangement with Leuven, he became professor in both universities from 1962, dividing his time between Leuven and New York. He became emeritus professor of the University of Louvain in 1985, and of Rockefeller in 1988.