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We describe 10 cases of B-cell non-Hodgkin lymphoma (NHL) that did not express immunoglobulin kappa or lambda light chains by dual-color flow cytometry. Cases were identified from 298 consecutive cases of B-cell NHL and included follicular center cell lymphoma, diffuse large B-cell lymphoma, small noncleaved cell lymphoma, and small lymphocytic lymphoma. One case did not express any immunoglobulin heavy chain (IgH) as well; however, isolated expression of IgG heavy chain was seen in another case. Immunoglobulin heavy chains were not part of the lymphoma panel in other cases. All 3 cases in which gene rearrangement studies were performed showed rearrangement of IgH genes, including the case that did not express surface IgH chains. Immunoglobulin kappa light chain genes were rearranged in 2 of 3 cases and were in germline configuration in the third. All 147 cases of benign lymph nodes analyzed by flow cytometry showed polyclonal expression of immunoglobulin kappa and lambda light chains. Because of the absence of surface immunoglobulin light chains, these tumors must be distinguished from precursor B-cell acute lymphoblastic leukemia, plasma cell tumors, and rare cases of florid follicular hyperplasia that do not express surface immunoglobulins. The absence of immunoglobulin expression on malignant B cells can result from defects at any level from gene transcription to translocation of fully assembled proteins to the cell surface.