Analysis of the morphological and immunologic [surface (Sm Ig) and cytoplasmic (Cy Ig) immunoglobulin, faintness (f.imfl) or brightness (b.imfl) of surface immunofluorescence, rosette formation with mouse erythrocytes (MR), or sheep red cells coated with C3 (EAC)] characteristics of the neoplastic lymphocytes involved in 137 cases of B-cell leukemias allowed the differentiation of the following cytologic categories: B1 small lymphocytes (71); SM Ig+, f.imfl., Cy Ig-, MR+, EAC+; B2 prolymphocyte (4): SM Ig+, b.imfl., Cy Ig-, MR+, EAC+. B3 plasmacytoid lymphocyte (2): Sm Ig+, b.imfl., Cy Ig+/-, MR+!-, EAC+/- B4 small cleaved lymphocyte (23): Sm Ig+, b.imfl., Cy Ig-, MR-, EAC+. B5a large cleaved and B5b noncleaved lymphocytes (14): Sm Ig+, b.imfl., Cy Ig-, MR-, EAC-. B6 small noncleaved "Burkitt-like" lymphocyte (5): Sm Ig+, b.imfl., Cy Ig-, MR-, EAC-. B7 plasma cell (2): Sm Ig+, MR-, EAC-. B8 hairy cell (16): Sm Ig+, b.imfl., Cy Ig-, MR-, EAC-, also exhibiting ingestion or attachment of particulate material, not seen in other types. Improved delineation of the heterogeneous group of B-cell leukemias might be of developmental significance in lymphocyte differentiation and improve current prognostic and therapeutic criteria.