A sensitive cytofluorometric technique, the “kappa-lambda test,” permits detection of small numbers of monoclonal B lymphocytes (clonal excess). Such a method might represent a new diagnostic tool for diagnosis of non-Hodgkin’s lymphomas, potentially providing definitive evidence of lymphomatous involvement in cases equivocal by standard immunologic methods. To determine the significance of detecting B cell clonal excess in lymphoid tissues, we applied the kappa-lambda test to cell suspensions from 60 consecutive specimens suspected to involvement by malignant lymphoma. Results were correlated with the pathologic diagnosis and with standard cell marker studies in each case. B cell clonal excess was observed in 24 of the 25 cases of non-Hodgkin’s lymphoma of B cell origin, including a single case involving early detection of recurrence. None of the remaining cases, including benign reactive hyperplasia, T cell lymphoma, and Hodgkin’s disease, showed evidence of B cell clonal excess. Selective examination of cell subpopulations was also achieved using this cytofluorometric method. We conclude that the detection of B cell clonal excess by the kappa-lambda test represents a new approach to the diagnosis of B cell lymphoma, which provides certain advantages over more standard methods of cell marker analysis.