Cultures of marrow mononuclear cells were exposed to medium derived from cell cultures infected with several different strains of human herpesvirus-6 (HHV-6) immediately before the addition of either of two growth factors, granulocyte-macrophage colony-stimulating factor and interleukin-3. Exposure to any of the viral preparations suppressed the outgrowth of nonspecific esterase-positive adherent macrophages induced by the factors by more than 90%. The nonadherent cell populations in the infected cultures were numerically similar to those in uninfected control cultures, demonstrating the absence of a nonspecific cytotoxic effect of the viral materials. Infectious virus was not necessary for the macrophage outgrowth suppression. These findings suggest that HHV-6 either encodes or induces a soluble mediator or mediators that can interfere with the responses of bone marrow to growth factors and possibly block the normal differentiation of macrophages from marrow precursors.