Long-term tissue cultures derived from the TA3 ascites tumor of the mouse became nontransplantable but induced a high degree of immunoprotection against challenge with as many as 106virulent cells. Intraperitoneal immunization proved more effective than the intravenous or intramuscular routes. After chemical and physical treatments virulent TA3 cells showed no immunoprotective capacity. After similar treatments the nontumorigenic TA3 tissue-culture cells showed a moderate immunoprotective capacity. Nontumorigenic cells of the 6C3HED and TA3 lines induced a high degree of immunoprotection against solid tumor development produced by the subcutaneous injection of as many as 107virulent cells. Protection against solid tumor development could be demonstrated provided immunization with nontumorigenic cells was begun within 2 weeks of implantation of the virulent cells.