Cotransplantation of human fetal thymic tissue and CD34(+) fetal liver cells in nonobese diabetic (NOD)/severe combined immunodeficiency (SCID) or NOD/SCID/γc(-/-) mice results in the development of multilineage human hematopoietic cells. In this study, we show that these humanized mice had extremely low levels of human platelets. The presence of human megakaryocytes at a normal concentration in the bone marrow suggests that human megakaryocytic differentiation occurred efficiently in these mice. Rapid increase in human platelets in blood to levels comparable with those of human peripheral blood mononuclear cells (PBMCs) after macrophage depletion indicates that mouse macrophages are responsible for the poor human platelet reconstitution in humanized mice. In support of this possibility, human platelets were rapidly rejected after infusion into untreated mice, but persisted in macrophage-depleted mice. These findings indicate that inhibition or depletion of recipient mouse macrophages may provide a useful means for evaluating human thrombopoiesis and platelet function in vivo using immunodeficient mice.