We studied the effects of adding cytomegalovirus (CMV) in vitro to normal human bone marrow mononuclear cells (BM-MNCs), committed myeloid progenitor cells, primitive myeloid blast-colony forming cells, and pre-formed marrow stromal cell monolayers in order to shed light on the mechanism by which hemopoiesis is suppressed in patients who acquire systemic CMV infection after allogeneic bone marrow transplantation. Incubation of BM-MNCs or committed progenitor cells with laboratory strain AD169 or wild strain CMV had no significant effect on total colony numbers or the morphology of component cells. CMV mRNA was not identified by in situ hybridization. In contrast, incubating marrow stromal monolayers with CMV produced specific cytopathic effects in fibroblasts and adipocytes and reduced the capacity of the stromal layers to support the proliferation of primitive myeloid progenitor cells. We conclude that CMV infection may impair hemopoiesis in vivo by a direct effect on the cellular components of the marrow stroma.