The cause of aplastic anemia associated with hepatitis (AAH) is as yet still unknown. There is a supposed relation to the immune mechanisms, however few reports have shown the effects of monocytes on the pathogenesis in the patients with AAH. We have reported a case of a 12-year-old boy with AAH related to cytomegaloviruses, and studied the hemopoietic progenitors. He showed pancytopenia and hypoplasia of the bone marrow on admission to our hospital. The culture studies showed that mononuclear cells (MNC) of the bone marrow produced few hemopoietic colonies in all cell lineages. However, the depletion of adherent cells from the MNC increased numbers of erythroid, neutrophil-macrophage and megakaryocyte colonies. Furthermore, the addition of adherent cells of the peripheral blood suppressed the colony formation in the aforementioned cell lineages by marrow MNC from which adherent cells, phagocytic cells and T-cells were abrogated. The results way suggest that monocytes play some soles in the pathogenesis of aplasia through inhibitor of hemopoiesis.