The effects of cytomegalovirus (CMV) infection on hematopoietic progenitor cells in vivo were investigated to elucidate the pathogenesis of CMV-induced myelosuppression. BALB/c mice were inoculated with 0.2LD50 of murine CMV (MCMV). Lineage marker negative, c-kit positive (Lin-c-kit+) and Lin-CD34+ cells, which are both phenotypically defined as hematopoietic progenitor cells, showed a significant reduction in number on day 3 postinfection (pi). Moreover, the reduction in the number of day-14 colony-forming units-spleen (CFU-S), another indicator to identify hematopoietic progenitor cells, was noted on day 3 pi. To clarify the mechanism of such depletion, we examined the cells undergoing apoptosis in the Lin- populations and found a 15-fold increase in the apoptosis-induction of these cells. Furthermore, an increase in the expression level of Fas, which mediates apoptosis, was observed in such Lin-c-kit+ and Lin-Sca-1+ cells on day 3 pi. In vitro treatment with the anti-Fas antibody accelerated the apoptosis in Lin- cells, but not in the uninfected control cells, thus indicating that the upregulated Fas on Lin- cells is directly related to the acceleration of apoptosis found in these cells in vivo. These results suggest that MCMV infection reduces the number of hematopoietic progenitor cells in bone marrow at least in part due to Fas-mediated apoptosis, and this phenomenon is thus considered to contribute to CMV-induced myelosuppression.