The entry of Chlamydia species into host cells in culture and their subsequent fate will be studied with respect to the following topics: (1) The structural differences between the phagosomal membranes surrounding chlamydiae and ordinary phagosomal membranes that are responsible for the inhibition of the fusion of lysosomes with chlamydia-containing phagosomes. (2) The mechanism that controls conversion of cryptic infection to overt infection in mouse fibroblasts (L cells) persistently infected with C. psittaci (strain 6BC). (3) The property of the variant of C. trachomatis (strain G-17) that arises after prolonged contact of these chlamydiae with mouse fibroblasts (McCoy cells) that enables it to establish a state of persistent infection in these host cells.