Five- to eight-weeks old A.M.B. turkeys cannot be infected by intramuscular injection with cell-free Rous tumour agent unless they have been made tolerant by intravenous inoculation at 1 day old with whole blood or with washed blood cells from young normal chickens of the strain in which the sarcoma is usually propagated. The duration of the immunologically null period in the turkey poult has been investigated together with the specificity and stability of the tolerance-producing antigens and their relationship to skin transplantation antigens. The results afford strong support for the existence of at least one immunologically distinctive host component for the Rous agent, and this method of investigation, i.e. the production of virus tolerance in another species with identifiable antigens (possible sources of which are discussed), appears to offer an opportunity for identifying such a component or components.