The nature of rice bodies was studied, utilizing histochemistry, immunofluorescence, and scanning and transmission electron microscopy. Rice bodies were found to consist primarily of fibrous material, most of which was fibrin with small amounts of collagen. Channels containing a variety of viable cells permeated the rice bodies. Blood vessels occurred in a few rice bodies indicating a former connection with the synovial membrane. Nonvascularized rice bodies might represent a further degeneration of the vascular type. Rice bodies seem to be a nonspecific response to inflammation.