Subcellular preparations of mouse thymocytes were compared with respect to their power to raise in rabbits antisera which prolonged the survival of skin homografts. Of the fractions studied thymocyte `membrane' gave the most consistently good results, whereas thymocyte `mitochondria' gave rise to sera which were toxic in high dose. Whole mouse liver homogenate or liver subcellular fractions did not elicit potent antisera. It is concluded that thymocyte `membrane' best fulfils the requirements for an ideal material for immunization thus far studied and provides a convenient starting point for further purification.