The effect of methotrexate (MTX) on proliferation and on interleukin 1 stimulated secretory activities of human synovial fibroblasts in culture was investigated. MTX caused a dose dependent inhibition of growth over the concentration range 0.07-2.2 microM with a half-maximal effect at 0.37 microM. INhibition was competitively relieved by coaddition of leucovorin. Cell growth was fully restored after MTX pretreatment of 24 h but not after 48 h, even on subsequent leucovorin addition. Cell viability was unaffected by MTX treatment. MTX had no effect on interleukin 1 stimulated production of prostaglandin E, hyaluronic acid and collagenase. Our results raise the possibility that one of the mechanisms contributing to the therapeutic effects of MTX in patients with rheumatoid arthritis may involve modulation of synovial fibroblast growth.