The effect of indomethacin and phenyl butazone on hyaluronic acid production and pro liferation rate of human synovial cells in mono Phenylbutazone layer cultures was studied. de pressed the metabolic activity and in higher con centration also the growth rate of the cells. An inhibitory effect on hyaluronate production by indomethacin was inconstant. In no case indomethacin depressed cell proliferation. There were no differences in the drug response of the cells whether they were derived from patients with rheumatoid arthritis or nonrheumatoid articular diseases.