The aim of this study was to compare the results of flow cytometric (FCM) determination of heavy and light chain cytoplasmic immunoglobulin (cIg) with those obtained by the peroxidase-antiperoxidase (PAP) method. Fifty-one patients, including five non-T-acute lymphoblastic leukemias, 16 B-chronic lymphocytic leukemias (CLL), 13 non-Hodgkin's lymphomas, seven hairy cell leukemias, four multiple myeloma/plasma cell leukemias, and six T-cell leukemia/lymphomas, as well as 12 normal controls, were studied. Saponin-permeabilized cell suspensions were indirectly stained with monoclonal antibodies and analyzed by flow cytometry. Acetone-fixed cytocentrifuge smears were stained for cIg by the PAP method. The results obtained indicate that: (a) detection of cIg by FCM is a feasible and useful technique to confirm the B-cell lineage of leukemias and lymphomas, particularly those characterized by low-density surface immunoglobulin, such as CLL; and (b) cIg detection by FCM and PAP staining are complementary methods to recognize with certainty the monoclonality of B-cell malignancies.