Endocytosis, the internalization and processing of extracellular fluid and macromolecules, is a basic cellular process that is used by the cells of most organisms for nutrient uptake and for the recycling of membrane. Endocytosis is also used to modulate many biological processes ranging from synaptic vesicle recycling to receptor down regulation and antigen presentation. Endocytosis is exploited by pathogens for entry into cells, and numerous human diseases result from defects in this process. Furthermore, new treatment strategies utilize endocytosis as a vehicle to deliver therapeutic substances. A more detailed understanding of endocytosis will suggest novel treatments and applications through modulations of its pathways. The coelomocytes of Caenorhabditis elegans are scavenger cells that use conserved mechanisms to continuously and nonspecifically endocytose fluid from the body cavity. We identified 14 genes in a screen for mutations that disrupt endocytosis by these cells. Five of these genes have been cloned and encode novel proteins that are conserved in humans but are not found in yeast. Functional analysis of two of the mammalian homologues of these proteins has shown that they perform a similar function in mammalian cells. This demonstrates that studies in C. elegans will be important for discovering new proteins that function in endocytosis in animal cells and for defining their function. Since it is likely that the remaining genes also encode novel proteins that have a conserved function in animal cells, we propose to expand our analysis of this process in C. elegans by: 1) Determining the exact role in endocytosis of the genes we have already cloned; 2) Cloning the remaining nine genes we have identified and analyzing their function; and 3) Using the powerful tools of classical and molecular genetics in C. elegans to determine how the proteins that function in endocytosis interact together to regulate this process. Our long-term goal is to understand in detail how such a complex process functions and is regulated in animal cells. New genes identified in worms will also be studied in humans through collaborations with researchers who study this process.