Routinely stained sections of bone marrow core biopsy specimens obtained from patients with (A) T-cell acute lymphoblastic leukemia and (C) early T-cell precursor acute lymphoblastic leukemia demonstrate sheets of medium-sized blasts. Immunohistochemical studies performed with anti-BCL-2 antibody (clone 100/D5, Leica/Novocastra, Buffalo Grove, IL) on formalin-fixed, paraffin-embedded tissue of bone marrow core biopsy specimen of (B) the patient with T-cell acute lymphoblastic leukemia demonstrate that blasts show variable BCL-2 expression. The BCL-2 expression of blasts was weaker than the BCL-2 expression on intermixed small mature lymphocytes (arrows). In contrast, (D) the blasts of the patient with early T-cell precursor lymphoblastic leukemia demonstrated uniformly strong BCL-2 expression. Magnification, ×500.