Monoclonal antibody FMC7 detects subgroups of B-cell leukemias that have arisen from cells in late stages of B-cell maturation. FMC7 was studied by flow cytometry on cell samples from 192 patients with a diagnosis of chronic lymphocytic leukemia (CLL) or lymphoma. The leukemic cells from 16 patients were reactive with this antibody. These 16 cases were evaluated for other surface markers, morphology of cells, and clinical characteristics. Of the 16 patients, 14 had cells that strongly expressed surface immunoglobulin (SIg). This is atypical of CLL cells, which characteristically show weak expression of SIg. Eleven cases had kappa and five had lambda light chain. All patients' cells had consistently brighter CD20 expression than that of CD19. Fourteen patients had expression of CD5 on their leukemic cells. One patient had more than 55% prolymphocytes, meeting the criteria of prolymphocytic leukemia (PLL), two patients had CLL in prolymphocytic transformation (CLL/PL), and two other patients were classified as having a paraimmunoblastic variant of small lymphocytic lymphoma based on a high number of paraimmunoblasts and on the histologic features. Another nine patients had immature lymphoid cells distinct from prolymphocytes or paraimmunoblasts on morphologic study. The immature cells were variable in size, and the nuclear chromatin was less clumped than that of prolymphocytes. The histologic diagnoses in four of these cases were consistent with mantle cell lymphoma. Splenomegaly was observed in 11 patients (69%), and 11 patients had advanced Rai 3 or 4 disease. Among 10 patients treated with fludarabine, five responded to therapy. Monoclonal antibody FMC7 is useful for identifying a group of atypical variants of CLL, PLL, and other B-cell lymphomas in leukemic phase that can be easily confused with CLL. Careful attention to the cell morphology and histologic features is important for the differential diagnosis of FMC7-positive, B-cell lymphoproliferative diseases.