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The effects of soluble E-selectin, P-selectin and normal platelets on acute myelogenous leukaemia (AM L) blasts were investigated in vitro. We investigated effects on spontaneous and cytokine-dependent blast proliferation, and constitutive blast secretion of different cytokines. The presence of normal platelets during in vitro culture caused a dose-dependent increase in both spontaneous and cytokine-dependent AML blast proliferation. Addition of platelets also increased constitutive blast secretion of Interleukin 1beta (IL1beta ), IL6, GM-CSF and TNFalpha, whereas platelets had no effect on the release of IL1 receptor antagonist. The effects of platelets on constitutive cytokine secretion were also detected when platelets and AML blasts were cultured in different chambers separated by a permeable membrane, and a further enhancement was achieved when blasts and platelets were cultured together. Soluble P-selectin had no effect on constitutive AML blast cytokine secretion or the platelet-induced enhancement of the secretion. However, both soluble E- and P-selectin altered AML blast proliferation for a minority of patients. We conclude that normal platelets can modulate the function of human AML blasts in vitro.