Defects of the bone marrow microenvironment have been implicated in the pathogenesis of aplastic anemia (AA). We examined granulocyte/macrophage colony-stimulating factor (GM-CSF) and IL-6 production in bone marrow fibroblasts from five children with AA in response to cytokines and viral challenge with cytomegalovirus (CMV). Cytokine-stimulated IL-6 production in patients' fibroblasts was significantly reduced, whereas GM-CSF production was found to be comparable to normal controls. Also, CMV-mediated stimulation of IL-6 production was significantly lower in bone marrow fibroblasts from patients with AA. During the late phase of CMV-infection, IL-6 production was suppressed in CMV-infected fibroblasts from patients with AA with 90% inhibition of IL-6 protein production and marked reduction in IL-6 mRNA accumulation. Defective IL-6 production in bone marrow fibroblasts might contribute to hematopoietic suppression in some patients with AA.