Adjuvant-arthritis was induced in rats by means of a subplantar injection (left hind paw) of M. butyricum in paraffine oil and the severity of the disease determined volumetrically on the right, uninjected hind paw when the secondary lesions had reached near-maximal extent. One group of the rats was treated on days 18 to 21 post-adjuvant with hydrocortisone at a daily oral dose of 30 mg/kg and the therapeutic effect assessed in terms of gross paw-oedema changes and of certain parameters mentioned hereafter Using a variety of ‘clinical parameters’ (ESR, albumin/globulin ratio, fibrinogen, fibrinolysis, serum iron and serum copper) as well as other parameters still under experimental evaluation (peritoneal mast cells and ‘inflammatory’ rest cells) it was found that: (a) the ‘clinical parameters’ undergo much the same changes in adjuvant-induced arthritis in the rat as they do in human rheumatoid arthritis; (b) steroid therapy reverses such changes to normal in the rat in a fashion similar to that obtained in man suffering from rheumatoid arthritis and the like; (c) peritoneal mast cell count is low and peritoneal ‘inflammatory’ rest cell count high in full-blown adjuvant-arthritis; (d) a short-term steroid therapy tends to normalise the changes mentioned under (c).