In this study the apoptotic effects of heparin on lymphoblasts, neutrophils, and mononuclear cells were evaluated by flow cytometry for detection of sub G(1) peak, in vitro. Ten children with acute lymphoblastic leukemia (ALL) at diagnosis (Group I), six children with ALL at relapse (Group II), and 10 healthy children (controls) were included in this study. Lymphoblasts in ALL patients, and neutrophils and mononuclear cells in controls, were incubated in increasing heparin concentrations (0, 5, 10, 20 U/ml), Flow cytometric analyses were performed at 0, 1, and 2 hours of incubation in heparin for determination of the apoptotic effects of heparin. In Group I apoptosis was detected in all different levels of heparin concentration except 0 U/ml at 0, 1, and 2 hours, The apoptotic effects of heparin on blast cells peaked at the first hour in 5-, 10-, and 20-U/ml heparin concentrations (p < 0.0001), In Group II similar findings were observed only at zero hour and apoptosis was higher than those in Group I except in 5-U/ml heparin concentration (p < 0.001). Apoptosis was found to increase with heparin levels in both groups (p < 0.02). In the control group, apoptosis was detected only at the 20-U/ml heparin concentration and only at the first and second hours, Lymphoblasts are more sensitive to apoptotic effects of heparin than either neutrophils and mononuclear cells (p < 0.004). It can be suggested that low-dose heparin may cause significant apoptosis of lymphoblasts while inducing no apoptosis on neutrophils and mononuclear cells. The findings of this preliminary study indicate that further and more comprehensive research on the apoptotic effect of heparin on lymphoblasts should be done. Am. J, Hematol, 61:90-93, 1999, (C) 1999 Wiley-Liss, Inc.