Antiidiotype (Id) antibodies identify unique determinants within the surface immunoglobulin (Ig) that are present on B-cell tumors. Anti-Ids have been used for diagnosis and therapy of B-cell lymphoma and leukemia. A panel of 29 anti-Id monoclonal antibodies (MoAbs) that recognize shared idiotypes (SIds) on B-cell lymphomas was tested for reactivity with both B-cell leukemias and lymphomas. Ten of 40 (25%) cases of chronic lymphocytic leukemia (CLL) reacted with at least one of the 29 anti-SId MoAbs. Three cases reacted with more than one anti- SId MoAb, but there was no repetitive pattern of a single anti-SId MoAb reacting with a large proportion of CLL cases. In contrast, for B-cell lymphoma, in which 11 of 31 (36%) cases reacted, one anti-SId (B4–1) reacted with five of the positive cases; all were diffuse histology. Restricted anti-SId reactivity may lead to important insights into the etiology of certain B-cell lymphomas. In addition, these anti-SIds may obviate the need to develop “tailor-made” antibodies for individual patients.