Attempts were made to transfer tuberculin sensitivity from man to monkey (Macaca mulatta) and from man to rabbit by means of leucocytes obtained from the peripheral blood of specifically sensitive human donors. Despite variations in the route of administration (intradermal and subcutaneous, intraperitoneal, and intravenous injection of leucocytes) and in the method of treatment of the leucocytes prior to transfer (intact, or frozen-thawed and deoxyribonuclease-treated leucocytes, with and without incubation with antigen), the transfer recipients remained tuberculin negative upon subsequent repeated skin testing. Within the limitations of the experimental approach employed, it would appear that it is not possible to transfer delayed hypersensitivity to tuberculin to the monkey or the rabbit by means of leucocytes obtained from the peripheral blood of sensitive human donors.