The plague is caused by the bacteriumYersinia pestis. Plague bacteria are thought to inject effector Yop proteins into host cells via the type III pathway. The identity of the host cells targeted for injection during plague infection is unknown. We found, using Yop β-lactamase hybrids and fluorescent staining of live cells from plague-infected animals, thatY. pestisselected immune cells for injection. In vivo, dendritic cells, macrophages, and neutrophils were injected most frequently, whereas B and T lymphocytes were rarely selected. Thus, it appears thatY. pestisdisables these cell populations to annihilate host immune responses during plague.