Studies on lysosomal activation of Chlamydia psittaci infected L cells have been performed during the growth cycle of the agent and extended for 4 days after the initiation of infection. The present results indicate that lysosomes of infected L cells were not injured during phagocytosis, but that lysosomal enzyme release first occurred about 24 h after infection and apparently preceded the death and lysis of host cells. These observations attach a new significance to the study of injury and death of chlamydial infected cultured cells as well as to studies on the pathogenesis of psittacosis at the whole host level.