Radioactive particles as well as X‐irradiation from an external source have been used in the treatment of rheumatoid arthritis, osteoarthritis, and ankylosing spondylitis. In order to clarify effects of ionizing irradiation on synovial cells, radioactive gold (198Au) and yttrium (90Y) were added to fibroblast cultures derived from human synovial membranes. Other cultures were irradiated by a Picker X‐ray machine. Fibroblast growth and hyaluronic acid production were measured. Radioactive gold and yttrium particles induced a significant increase of hyaluronic acid synthesis rate (pg/cell/day) and inhibited fibroblast growth. Fibroblasts continued to overproduce hyaluronic acid and to show growth inhibition 3 weeks after irradiation with radioactive gold. Hydrocortisone inhibited hyaluronic acid overproduction induced by radioactive gold. Overproduction of hyaluronic acid induced by the X‐ray machine was inhibited by hydrocortisone, actinomycin‐D, and cyclo‐heximide. Fibroblasts derived from normal and rheumatoid patients responded similarly to ionizing irradiation.