The tumorigenic properties of human rheumatoid arthritis synovial cells in culture were investigated. The synovial cells developed good colonies and secreted plasminogen activator (PA) and collagenase in the cell cultures, as do Hela cells. Since PA and progesterone receptor (PgR) are considered to be end products of estradiol action in breast cancer cells, the estrogen receptor (ER) and PgR content in these cells was also assayed. Large amounts of ER and PgR were detected in the synovial cells in culture, even though these cells are not targets for sex steroids. Study of the cytomorphologic changes in the synovial cells in culture revealed many characteristics generally observed in neoplastic cells. Whether any or all of these observations have any implication in prognosis or therapy in this disease remains to be studied.