The effect of anti-inflammatory steroids (prednisolone and derivatives of hydrocortisone, dexamethasone and betamethasone) on the synthesis of hyaluronic acid and sulphated glycosaminoglycans in human skin fibroblast culture was studied. The concentrations of steroids varied between 1 x 10(-10)M and 1 x 10(-6)M. All tested steroids decreased the synthesis of hyaluronic acid to the same final level which was about 40--50% of the controls, but the concentrations required varied between different steroids. The relative inhibitory potencies of the steroids were calculated based on concentrations needed to decrease the synthesis of hyaluronate. When the inhibitory potency of hydrocortisone was calculated as one, the values of the other steroids were: prednisolone 5, hydrocortisone 17- butyrate 20, betamethasone alcohol 30, dexamethasone alcohol 38, betamethasone 17-valerate 350--400, dexamethasone monosodium phosphate and betamethasone disodium phosphate over 400. Hydrocortisone sodium succinate was as potent an inhibitor of hyaluronate synthesis as hydrocortisone alcohol. None of the tested steroids affected the synthesis of sulphated glycosaminoglycans at these concentrations. The changes ovbserved in glycosaminoglycans in the medium were in accordance with the changes in the cell layer. The possible significance of hyaluronate synthesis inhibition by anti-inflammatory steroids is discussed.