Plasma cells are widely distributed in inflamed periodontal tissues, the adjacent periodontal ligament and nearby alveolar bone spaces, of old rats (20 and 27 months) raised on a conventional diet in a normal laboratory environment. Electron microscopy revealed three morphologic types (or perhaps stages) of these cells based on the arrangement and content of the granular endoplasmic reticulum. Close contact between plasma cells and other cell types, such as lymphocytes, macrophages and fibroblasts, were commonly observed. It is suggested that plasma cell infiltration is a widespread and prominent feature of naturally occurring periodontitis in old rats, resembling the condition known to exist in humans, monkeys and dogs. The occurrence of large numbers of apparently fully differentiated plasma cells in otherwise normal alveolar bone marrow is discussed.