Flow cytometric detection of CD25, but not CD2, expression on bone marrow mast cells from patients with HES and elevated serum tryptase levels. Multicolor flow cytometry results are shown for CD117⁺ high, side-scatter high bone marrow cells from a representative patient with systemic mastocytosis and eosinophilia (A-D) and a patient with HES and elevated tryptase (E-H). Cells were stained with FITC-conjugated mouse IgG1 control antibody (A,E), antihuman CD2 antibody (B,F), antihuman CD25 antibody (C,G), and antihuman CD35 antibody (D,H). CD117⁺ high, side-scatter high bone marrow mast cells are shown in red.