Professor Stephen Boyden is distinguished for his contributions to immunology, both at the experimental and the conceptual level. In 1951, Boyden introduced the "tanned red cell" method of titrating antibodies, which is the most sensitive and possibly the most widely used method of titrating antibodies to proteins. Beween 1952 and 1958,, he was concerned particularly with the immunology of tuberculosis, on which he is a recognised world authority. Since then his interests have broadened to encompass three fields, immune responsiveness, the cellular recognition of foreign material, and delayed-type hypersensitivity. His discovery of cytophilic antibody may represent a most important advance in the understanding of the phenomenon of delayed-type hypersensitivity.