One hour after suspensions of mouse fibroblasts (L cells) were exposed to 500 to 1,000 L-cell 50% infectious doses of Chlamydia psittaci (6BC), the L cells failed to attach to and spread out on solid substrates, phagocytosed polystyrene latex spheres at reduced rates, incorporated less [14C]isoleucine into protein, and had smaller soluble pools of nucleoside triphosphates. The infected L cells began to die at 8 h and were all dead by 20 h. Lower multiplicities of infection took correspondingly longer to kill the L cells. These effects of high multiplicities of C. psittaci on L cells will be referred to collectively as immediate toxicity. Similar effects were obtained with other strains of C. psittaci and C. trachomatis and with other cell lines. Ultraviolet-inactivated C. psittaci retained the ability to cause immediate toxicity, but heat-inactivated chlamydiae did not. C.psittaci cells had to be ingested by L cells before they were immediately toxic but, once they were phagocytosed, they did not need to multiply or to synthesize macromolecules in order to cause immediate injury to their hosts. Immediate toxicity was not the result of depression of energy metabolism, changes in the levels of intracellular cyclic nucleotides, or release of hydrolases from lysosomes. It was suggested that a lesion is produced in the plasma membrane of the L cell every time it ingests a chlamydial cell, that each act of ingestion produces an independent lesion, and that their injurious effects are additive. Thus, the more ingestion lesions there are, the faster the host cell dies. It was also suggested that induced phagocytosis, inhibition of lysosomal fusion, and death of mice and of cells in culture may all depend on a single activity of C. psittaci.