Most antibody panels proposed for flow cytometric immunophenotyping of non-Hodgkin's lymphomas and chronic lymphoid leukemias include anti-CD20 and FMC7 antibodies. As in our experience, reactivity of B-cells with these antibodies seemed to be correlated, we evaluated whether the simultaneous use of anti-CD20 and FMC7 antibodies is justified. Using flow cytometry, we measured the binding of these 2 antibodies to the B-cells of 67 bone marrow aspirates, 31 lymph node biopsies, 18 peripheral blood specimens, and 12 tissue samples from other locations. The diagnoses included 50 cases without overt abnormalities, 5 reactive lymphadenopathies, 56 lymphomas and chronic lymphoid neoplasias, and 17 cases with other malignancies. Although CD20 expression was consistently higher, we observed a significant and strong correlation between CD20 and FMC7 antigen expression on B-lymphocytes, irrespective of the nature of the sample or disease (r ؍ 0.910; P Ͻ 0.001). Moreover, FMC7 antigen expression on B-cells could be predicted by CD20 expression with a sensitivity of 96%, a specificity of 94% and an efficiency of 96%. Our results show that although differing in intensity, expression of CD20 on B-cells closely parallels that of FMC7 antigen. We, therefore, conclude that little additional information is revealed by using FMC7 in immunophenotyping of non-Hodgkin's lymphomas or chronic lymphoid leukemias if intensity of CD20 expression is taken into consideration.