Yersinia pestis, the infective agent of bubonic and pneumonic plague, is classified as a category A agent of bioterrorism. YopM, a protein toxin of Y. pestis, is necessary for full virulence in a systemic plague mouse model with ambient-temperature grown bacteria. We used this model to identify the cells undermined by YopM. Natural killer (NK) cells were previously reported to be lost in spleen and blood in a YopM-associated way; however, NK cell depletion was found not to occur in liver, and ablation of NK cells had no effect on bacterial growth, indicating that NK cells are redundant for the YopM-mediated virulence mechanism. In mice either ablated for Gr1+ cells or lacking the chemokine receptor CCR2, wildtype growth was at least partially restored to the ΔyopM Y. pestis in both spleen and liver. In a mouse model of bubonic plague, CCR2 also was shown to be required for the ΔyopM Y. pestis to show wildtype growth in skin. These data pointed towards Gr1+CCR2+ cells, which include polymorphonuclear leukocytes (PMNs), inflammatory monocytes and inflammatory dendritic cells (iDCs), as key cellular components in controlling the in vivo growth of the ΔyopM Y. pestis. Recruitment of Gr1+CCR2+ cells into both organs was not YopM-dependent, except iDCs, whose influx from blood into spleen was blocked by the YopM-producing Y. pestis parent strain, but not the ΔyopM mutant. Ablation of PMNs, another main subset of Gr1+CCR2+ cells, relieved the growth limitation of the ΔyopM Y. pestis in liver, but not in spleen. Taken together, the data imply that YopM disables the recruitment of CCR2+Gr1+ iDCs into spleen but compromises the function of CCR2+Gr1+ PMNs in liver. These results were recapitulated when the infecting ambient-temperature-grown bacteria received 3 hours of incubation at 37°C. However, the appearance of the ΔyopM mutant phenotype was postponed at least 24 hours, indicating that some virulence property of Y. pestis that was induced by the temperature transition compensated for the loss of YopM in the ΔyopM strain at an early stage of the infection. In general, these findings provide a framework for focusing future research to identify YopM’s direct molecular targets.