Acute, sublethal infection of mice with murine cytomegalovirus (MCMV) resulted in up to 80% decreases in the number of cells recoverable from the bone marrow, and a decrease in peripheral blood leucocyte counts during the first week of infection. Depopulation of the leucopoietic areas of the marrow was evident from examination of histological sections. The severity of bone marrow atrophy in MCMV-infected mice of different strains correlated with previously described genetically determined sensitivity to MCMV disease. Although the phenomenon only occurred when mice were inoculated with infectious virus preparations, fewer than one in 10(5) marrow cells were productively infected, suggesting that atrophy was not due to lytic infection of large numbers of bone marrow cells. Interestingly, increases in serum colony-stimulating activity were observed and these were proportional to the severity of bone marrow atrophy. After MCMV infection, we observed increases in the proportions of cells expressing some B-cell and myeloid cell markers and a decrease in the proportion of cells expressing an erythroid cell marker. There was no change in the frequency of marrow cells expressing mature T-cell markers. The numbers of myeloid lineage-committed progenitor cells (GM-CFU) in the marrow decreased 10- to 20-fold in BALB/c nu/+ mice, while there was a threefold decrease in their numbers in BALB/c nu/nu mice. In addition, increases in serum colony-stimulating activity were greater in BALB/c nu/+ mice than in BALB/c nu/nu mice. Our results suggest that growth factors produced after MCMV infection may accelerate the maturation and migration of cells from the marrow to sites of virus replication and inflammation, thus accounting for the depletion in numbers of marrow cells observed soon after MCMV infection.