Studies continue to focus through use of in vitro techniques on three major groups of human pathogens with the objective of elucidating patho-physiological mechanisms and of developing new approaches to prevention of disease. In the area of human viral pathogens cytomegalovirus and varicella-zoster virus will receive emphasis. Studies on cytomegalovirus will focus on: a) Elucidation of natural history of infection in adults; b) Documentation that the agents are not one, but an antigenically related group of viruses; c) The nature of persistent infection at the cellular level; d) Examination of possible role of cytomegaloviruses in production of one type of human cancer. Work on varicella-zoster virus will seek a useful animal model and will focus on attempts to increase yields of infectious virus as a means of obtaining immunizing materials. The second major group of pathogens, the helminths, is represented by a project aimed at growing cells of Schistosoma mansoni in culture. Success in this area theoretically could eventually provide materials for a vaccine, would yield diagnostic antigens, and might assist in the search for new chemotherapeutic approaches. To date, no one has been able to culture cells of a helminthic parasite. Work on a third major group, pathogenic protozoa, is designed to elucidate the role of cellular immunity in Chagas' disease by investigating the mechanisms whereby macrophages from immune animals destroy intracellular forms of Trypanosoma cruzi.