Evidence has accumulated to indicate that two different populations of lymphocytes are responsible for immunologic responsiveness. The functional characteristics of these cells appear to differ; thymic-derived or T lymphocytes 3 are associated with cellular immune responses whereas B lymphocytes (probably bone marrow-derived or bursa-dependent) have been implicated in humoral aspects of the immune response (1). The isolation and characterization of T and B lymphocytes are essential for a full assessment of their functions. Recently surface markers on lymphocytes have provided a tool for identifying their origin. With direct immunofluorescent technique, lymphocytes with surface immunoglobulins have been shown in animals to be bone marrow- or bursa-derived, whereas lymphocytes of thymic origins are not stained by this technique (2–4). On the basis of this distinction it has been suggested that human peripheral blood lymphocytes bearing immunoglobulins are also B cells (5).