There are striking differences in the histogenesis of B and T cells. 1) The formation of B lymphocytes seems to be closely associated with that of other blood cells, whereas the formation of T cells is not. 2) In postnatal life, the bone marrow is a mojor site for the formation of B cells, while T cells are produced mainly in the thymus. 3) B cells are formed in a short production pathway, consisting of two or three mitoses, whereas T cells are formed in a long production pathway, consisting of about eight mitoses. 4) The starting point for the short production pathway is a large transitional cell. The long production pathway starts with a reticulum cell, a large lymphoblast or a cell which has been termed "monocytoid". 5) Plasma proteins have ready access to the cells of the short production pathway in bone marrow, but because of the blood-thymus barrier are not as readily accessible to the tissure involved in T cell formation in the long production pathway in the thymus.