Thymic-independent (B) lymphocytes, thymic-dependent (T) lymphocytes and histiocytes may be distinguished by the presence of certain surface markers. In addition B and T lymphocytes have been reported to show distinctive surface architecture by scanning electron microscopy. Neoplastic cells from a lymph node and cerebrospinal fluid of a patient with a diffuse malignant lymphoma of the poorly differentiated lymphocytic type were examined in frozen sections and cell suspensions for the presence of surface immunoglobulin and the antigen-antibody-complement (IgMEAC) receptor of B lymphocytes, the presence of the cytophilic antibody (IgGEA) receptor of histiocytes and the ability to form nonimmune rosettes with sheep red blood cells (E) characteristic of T lymphocytes. Cells from the lymph node were also studied by scanning electron microscopy. The majority of neoplastic cells from the lymph node and cerebrospinal fluid formed rosettes with E, but lacked surface immunoglobulin and failed to bind IgMEAC or IgGEA. By scanning electron microscopy the neoplastic cells, although larger in diameter, showed surface architecture similar to normal lymphocytes with a varying number of surface microvilli. These studies suggest that the malignant cells of this lymphoma are of thymic type.