Phagocytosis refers to the process by which cells are able to ingest large particles (>1 um). In vertebrates, phagocytosis mainly occurs in specialized cells of the immune system. In lower eukaryotes, phagocytosis provides a means in ingesting other microorganisms to meet the nutritional needs of the cell. Research in a number of experimental systems has made it clear that phagocytosis is a multistep process that involves hundreds of genes and proteins. Nonetheless, the identities and functions of phagocytic genes, and the molecular mechanisms of phagocytosis, are still poorly understood. As a result, we are analyzing the protein composition of phagosomes using the ciliate Tetrahymena as a model. Procedures for the purification of phagosomes from Tetrahymena have been developed. The proteins contained in such purified phagosome preparations are now being subjected to a global mass spectrometry analysis in collaboration with The Centre for Research in Mass Spectrometry at York University (Toronto, Canada). The approach involves subjecting total phagosome proteins to two fractionation steps prior to mass spectrometry. During the past year we have carried out a series of pilot analyses that have identified more >40 phagosomal proteins, and we have now refined the methods such that we expect to identity >125 proteins. We will also describe some of the tools available for Tetrahymena that will allow us to gain insight into the functions of the proteins identified in this analysis.