Triethylenethiophosphoramide ( T h i o-TEPA), the sulfur derivative of triethylenephosphoramide, has been found to be active in preventing metastases of rat mammary adenocarcinoma. 1 Clinically, temporary regression of human breast carcinoma has also been obtained, 2-4 this drug being one of the few so far discovered with a definite effect on malignant epithelial tumors. However, bone-marrow depression, especially in patients with osseous metastases, has been encountered as a serious toxic effect, limiting the dosage which can be employed. When marrow aplasia occurs in animals after exposure of the whole body to x-rays, the process can be reversed in some instances by an intravenous injection of homologous myeloid cells, the depleted blood-forming tissues being colonized by cells derived from those which have been injected. 5 The present experiments were therefore undertaken to determine whether homologous * marrow would similarly implant any proliferate when infused into rabbits which had received marrow-depressant doses of