The object of the proposed research program is a study of the mechanism of the selective transvascular passage of mature myeloid cells from their extravascular site of formation into the intravascular pool and of the selective transvascular passage of the lymphocytes (T cells and B cells) in their inter-organ transport between bone marrow, thymus and lymph nodes. The methodology includes: (1) light microscopic and electron microscopic observations on transvascular blood cell diapedesis in bone marrow and at extramedullary sites of hematopoiesis. (2) fine structural studies on the transvascular migration of blood cells in leukemia. (3) studies on the vascular permeability of the sinusoids in blood forming organs. (4) light microscopic and electron microscopic immuno-histochemical studies on the surface coat of mature and immature bone marrow cells and lymphocytes. (5) immuno-histochemical identification of T and B lymphocytes in bone marrow, lymphatic tissue and thymus and engaged in transvascular migration.