Many pathogenic bacterial species produce factors that promote their internalization by host cells. The crucial components for uptake of one such pathogen, Yersinia pseudotuberculosis, have been identified. Efficient uptake of this microorganism requires tight binding of the bacterial invasin protein to integrins on the cell surface. Internalization also involves coordination of signals responsible for cytoskeletal rearrangements and those involved in receptor-mediated endocytosis. A start is being made to define the proteins that are required for efficient completion of the internalization process.