The development of granulocytic hemopoiesis in the fatty marrow of metatarsal bones and caudal vertebrae of adult mice was studied in histological sections for up to six weeks following subcutaneous innoculation of granulocytosis inducing mammary carcinoma. The initial events observed were increase and engorgement of fatty marrow sinusoidal vascular beds, accompanied by numerous lymphoid mononuclear cells in the sinusoids and in the stroma. Foci of granulocytopoiesis appeared admixed with lymphoid cells in the stroma and near the endosteum. Hyperplastic granulocytopoiesis eventually predominated in the marrow of these bones as well as the femoral and sternal marrow of tumor bearing mice. The morphological findings suggested the possibility of stem cell and progenitor cell migration into fatty marrow, but activation of dormant stem cells could not be ruled out. The prevalence of granulocytopoiesis in the entire skeletal marrow in tumor bearing mice may seriously impair the production of other cellular components of the marrow including lymphocytes, reduced production of the latter would imply serious compromise for the immune system of the tumor bearing animals.