After periods in the United States and at Cambridge, he was appointed to the Joseph Hunter Chair of Pathology at the University of Sheffield in 1931. In 1935 he returned to Oxford, as Professor of Pathology and Fellow of Lincoln College, leading a team of researchers. In 1938, working with Ernst B. Chain and Norman Heatley, he read Alexander Fleming’s  1928 paper discussing the antibacterial effects of Penicillium notatum mould.