The capacity of a host to produce and mobilize monocytes is an essential component of host defenses during the early phases of infection caused by Listeria monocytogenes. In this study, the concentrations of colony stimulating factor (CSF) and the numbers of monocyte progenitor cells (CFUm) were measured in mice during infection with L. monocytogenes. The concentration of CSF in serum increased sharply during the first 24 h of infection and remained elevated for the next 7 days. The number of CFUm in the bone marrow, however, decreased during the first 4 days after injection of L. monocytogenes. Thereafter, the number increased slowly, returning to normal on day 14. The decrease in marrow progenitor cells did not appear to result from a reduced sensitivity to CSF. In contrast to bone marrow changes, spleen progenitor cells increased greater than 400%, reaching a peak 7 days after bacterial challenge. These data indicate that monocyte production during L. monocytogenes infection is correlated with a rise in serum CSF concentration, depletion of bone marrow CFUm, and an increase in the number of spleen CFUm.