Membrane-bound Ig were studied by immunofluorescence methods in 116 patients with lymphoproliferative disorders. Surface Ig synthetized in vitro were studied in many patients. In 70 of 73 cases of chronic lymphocytic leukemia (49 without and 24 with monoclonal serum Ig), monoclonal surface Ig with a distribution for heavy and light chains similar to that of normal lymphocytes were found. In three cases, these surface Ig accumulated as crystals in the lymphocyte cytoplasm. Studies limited to staining of freshly drawn cells may lead to erroneous conclusions since serum antibodies may become bound to the cell surface and simultaneous positivity for µ, γ, κ, and λ, due either to the attachment of immune complexes at the lymphocyte surface or to anti-IgG antibody activity of the monoclonal surface IgM, was not rare. IgM with anti-IgG activity was found on the surface of both lymphocytes and lymphoblasts in a patient with acute transformation of chronic lymphocytic leukemia. In a single case, µ and γ chains were simultaneously found on some lymphocytes besides two single producer clones. A biclonal proliferation characterized by distinct surface Ig markers was demonstrated in three other cases of lymphocytic leukemia and in a patient with Waldenström’s macroglobulinemia. In this latter condition (31 cases), most marrow lymphoid cells including plasma cells and the majority of blood lymphocytes carried IgM determinants which had the same light chain type as the serum IgM and which shared its eventual antibody activity. Similar results were obtained in a few patients with the hematological features of macroglobulinemia but with serum monoclonal IgG or IgA or with unreleased IgM. Studies on cases of γ and α heavy chain diseases, cold agglutinin disease, leukemic reticuloendotheliosis, and Sezary’s reticulosis are also recorded. The value of surface Ig as B cell markers in lymphoproliferative diseases is outlined.