Most C57BL/1 female mice normally reject male skin grafts. Tolerance to male skin grafts is induced in C57BL/1 female mice in most instances by the injection of cell-free tissue preparations obtained from spleen, liver, kidney, lung and heart. The potency of disrupted tissue preparations is generally well preserved by storage of tissue at deepfreeze temperatures for periods of 2–6 months. A notable exception to this observation in our experiments were the preparations of strored spleen. Tolerance to male skin grafts may be induced in the majority of C57BL/1 female mice by a single injection of 200–400 mg of cell-free antigenic material prepared from liver, spleen, kidney, heart and lung tissue as well as by the series of injections originally used which involved in aggregate 900 mg wet weight tissue equivalent. In the case of liver tissue, the administration of as little as 100 mg of cell-free antigenic material in a single injection has produced a high incidence of tolerance and a perceptible effect has been noticed at doses as small as 10 mg.