The role of lymphocytes in acute, non-immune inflammation has been studied in rats. Using drug-induced leucopenia and cell transfer studies it was found that the reduced carrageenin paw oedema in leucopenic rats could be restored by a specific subclass of lymphocytes distinct from T and B lymphocytes. The inflammatory effect of these lymphocytes appears to be mediated by a factor, the release of which is inhibited by steroidal and non-steroidal anti-inflammatory drugs and antilymphocyte sera.