The objective of this project is to study the role of GTPases of the Rho subfamily during phagocytosis in macrophages Interaction of microorganisms with cell surface receptors initiates signalling pathways, leading to submembraneous actin assembly and ingestion of the particle. Rho, Rac and Cdc42 are involved in the control of various cellular processes dependent on actin polymerization. First, We shall analyse the effect of inactivating these proteins on phagocytosis of IgG-coated particles. Next, using transfection and microinjection techniques we shall define how Rho, Rac and Cdc42 control signalling pathways linking ligation of Fc*R or CR3 to actin polymerization during phagocytosis in COS cells and U937 derived macrophages. Finally, we shall analyse the molecular pathways activated upon interaction between macrophages and different bacterial pathogens. These studies may allow us to identify intracellular targets for virulence factors of Yersinia spp. for example, which are able to impair actin polymerization and inhibit their uptake.