Diacerhein (DAR), a new drug which is particularly suitable for the treatment of osteoarthritis, was studied for its interference with the phagocytic capacity of cells coming from exudates of subcutaneous carrageenan oedema and from the peripheral blood of Sprague-Dawley rats. DAR was found to inhibit phagocytosis in both types of cells examined. This finding indicates that DAR may exert its action by means of a direct effect on the cells involved in the inflammatory process.