The need for a simple, reproducible and sensitive in vitro assay for cell-mediated immunity, or delayed-type hypersensitivity, in man is widely appreciated. In recent years understanding of the role of cell-mediated immunity in disease, and perhaps more importantly in health, has progressed remarkably owing to the success of in vitro approaches in laboratory animals. Clinically relevant questions that suitable in vitro tests could answer include the following: whether patients exhibit autoimmune reactivity to their own or transplanted tissue antigens; whether they have generalized or specific deficiencies in the delayed-type response; and, most critically, whether the patient with cancer has sufficient . . .