We investigated the significance of cytokines (soluble interleukin-2 receptor, granulocyte-macrophage colony-stimulating factor, interleukin-6, and interferon-gamma) and CD68-positive microparticles in immune thrombocytopenic purpura. Cytokines were measured by enzyme-linked immunosorbent assay and microparticles were detected by flow cytometry. CD68 expression by histiocytic U937 cells incubated with lipopolysaccharide or cytokines was also assessed in a control study. The level of CD68-positive microparticles was significantly higher in the patients with thrombocytopenia than in normal controls (p < 0.01). The soluble interleukin-2 receptor level was also significantly higher in patients than in controls (p < 0.01), but the other cytokines did not show a significant difference. However, patients with severe thrombocytopenia (platelet count > 20,000/microliters) had significantly higher levels of granulocyte-macrophage colony-stimulating factor and interleukin-6 than the controls (p < 0.05). When opsonized platelets were incubated with activated U937 cells, lipopolysaccharide and granulocyte-macrophage colony-stimulating factor caused an increase of CD68-positive microparticles in the supernatant. These results suggest that granulocyte-macrophage colony-stimulating factor is released by activated T cells in immune thrombocytopenic purpura and activates monocyte/macrophage phagocytosis, resulting in an increase of circulating CD68-positive microparticles and enhanced platelet destruction.