Infection with cytomegalovirus (CMV) continues to be one of the most common complications following allogeneic bone marrow transplantation. The gravest danger for the host occurs when the virus is reactivated as a result of immunosuppression. In this report we studied the effects of sublethal murine cytomegalovirus (MCMV) infection on the hemopoietic system including bone marrow (BM) cellularity, production of colony stimulating factor (CSF) and interleukin-6 (IL-6) and the development of granulocyte-macrophage colony forming units (CFU-GM), and BM stromal cell viability. Our findings show that the virus infection led to a significant decrease in the number of BM cells and in the production levels of CSF and IL-6. There was also a decrease in the number of stromal cells, as reflected by the number of colony forming unit fibroblasts (CFU-F), and in the relative number of CFU-GM progenitors. Treatment of MCMV infected mice with the immunomodulator AS101 [ammonium trichloro (dioxyethylene 0-0')tellurate] restored significantly CSF and IL-6 production by BM cells to levels of uninfected control mice as well as the number of CFU-F and stromal cell elements which consequently led to the restoration of the total number of BM cells. Results presented here indicate that AS101 may have immunomodulatory effects on MCMV mediated myelosuppression. Administration of AS101 to patients with CMV associated BM damage may improve the restoration of their BM function.