The effect of irradiating the transplantation site before the grafting of unirradiated tumor tissue (the tumor-bed effect, TBE) is shown to be dependent on the dose of x-yays, with a maximum at 3,000r. A variety of histological types of tumors are inhibited by pretransplant irradiation. The influence of sex, hormonal status, strain, and possible genetic difference on the TBE are minimal, but the nature of the transplantation site (subcutaneous tissue vs. brain) played a critical role in the TBE. The radiation-induced changes that give rise to the tumor bed effect persisted for at least nine months.