We have studied the effects of murine cytomegalovirus (MCMV) infection on bone marrow stem and progenitor cell populations to find an explanation for the defects in hematopoiesis that accompany CMV infections in patients. Sublethal MCMV infection of BALB/c mice resulted in a 5- to 10-fold decrease in the numbers of myeloid (colony-forming unit-granulocyte-macrophage [CFU-GM]) and erythroid (burst-forming unit-erythroid [BFU-E]) progenitor cells in the marrow, but not in primitive myeloerythroid progenitor cell (colony-forming unit-spleen [CFU-S]) numbers. In contrast, we observed a 10- to 20-fold reduction in CFU-S as well as CFU-GM and BFU-E in lethally infected mice. Depletion of marrow CFU-GM was less severe in C57BL/10 and C3H/HeJ mice, which are more resistant to the effects of MCMV infection. Treatment of bone marrow cells with MCMV preparations in vitro did not reduce the numbers of CFU-GM, although up to 10% of the cells were productively infected. This finding suggests that CFU-GM were not susceptible to lytic MCMV infection in vitro and are probably not eliminated by lytic infection in vivo. Increases in the frequencies of Sca-1+Lin- marrow cells, a population that includes cells with the characteristics of pluripotential stem cells, were observed in MCMV-infected BALB/c, C57BL/10, and DBA/2J mice. Increases in the frequencies of c-kit+Lin- marrow cells were only seen in DBA/2J mice. MCMV infection did not impair the function of pluripotential stem cells because transplantation of marrow from MCMV-infected donors into irradiated recipient mice resulted in successful reconstitution of the T, B, and myeloid cell lineages.