Publisher Summary This chapter discusses the small lymphocytes and transitional cell populations of the bone marrow. Their role in the mediation of immune and hemopoietic progenitor cell function is defined. In endothermic vertebrates the bone marrow is the central organ of hemopoiesis. It is the major source of all types of cellular elements that circulate in the blood. The bone marrow plays a major role in the development of those lymphocytes that function as the precursors of antibody-forming cells. In addition, it contains small lymphocytes competent to engage in cell-mediated immune responses. Moreover, the marrow can generate such cells under various experimental conditions. Lymphocytes of the marrow have also been implicated as pluripotent hemopoietic stem cells. On the whole, modern techniques of experimental hematology have furnished evidence in support of the historic argument, and now it seems clear that hemopoietic stem cells, and progenitor cells with varying degrees of commitment to specific lines of hemopoietic differentiation, are contained in the population of bone marrow cells designated by many investigators as “lymphoid.” The chapter examines the evidence that relates various biological parameters to the functions attributed to small lymphocytes and transitional cells.