The phagocytosis of Chlamydia psittaci and of trachoma and lymphogranuloma venereum biotypes of Chlamydia trachomatis by L cells, HeLa 229 cells, and mouse peritoneal macrophages will be studied by methods that independently measure attachment and ingestion. The extreme toxicity of chlamydiae for macrophages will be investigated. Quantitative chemical methods for following fusion of lysosomes with phagosomes will be developed and used to study inhibition of lysosomal fusion in chlamydia-infected cells. Subcellular fractions of host and parasite will be sought for that will reproduce the phenomenon of induced phagocytosis seen when chlamydiae react with non-professional phagocytes. The reasons for the limited infectivity of trachoma organisms for cells in culture will be investigated. The ability of lymphokines from sensitized lymphocytes antigenically stimulated in vitro to alter the interaction between chlamydiae and non-professional and professional phagocytes will be ascertained.