The role of bacterial infection in conventional mice in the regulation of granulopoiesis in vivo was studied. C57/bl 6J mice were injected i.p. with 1 .times. 104 Escherichia coli. The animals were evaluated for the following parameters: serum CSF [colony-stimulating factor], bone marrow CFU-C [colony-forming unit in culture], the absolute number of granulocytes in the peripheral blood and the number of bacteria in peripheral blood and abdominal cavity. Groups of 5 mice were investigated at multiple time points after injection. The results show peak levels of bacteria both in peripheral blood and abdominal cavity at the 1-h time point. There is a steady and marked rise in serum CSF reaching a 1st peak 8 h, a 2nd peak 96 h after the injection. The number of CFU-C is increased at the 48- and the 120-h time point. A pronounced rise in the absolute number of granulocytes is seen 2 h after infection, which seems to be independent of the activation of stem cells. A temporal relationship between the peak in the number of bacteria, release of mature granulocytes, rise in serum CSF and subsequent activation of bone marrow CFU-C was demonstrated. Following rises in CFU-C there are no marked elevations in peripheral blood granulocytes.