Abstract
McCoy cell cultures infected with the agent of ovine chlamydial polyarthritis were examined with the electron microscope. The agent was seen as small dense particles (250 to 450 nm) with an eccentric nucleoid and a multilaminated cell wall, as large (800 to 1,200 nm) granular particles surrounded by two unit membranes and as intermediate particles. Replication, which occurred throughout the cytoplasm, was initiated by phagocytosis of a small dense particle and terminated by rupture of the plasma membrane. Upon entering a cell, the small dense particles developed into large granular particles which divided by binary fission. Daughter particles either repeated the division or condensed to form new small dense particles.