To determine whether or not some growth factor (or factors) plays a significant role in the disease process of rheumatoid arthritis, the effects of pathological synovial extract, cultured synovial cell medium and partially purified extract and medium by fast protein liquid chromatography on cultured rabbit chondrocyte proliferation were studied. The synthesis of DNA was investigated by incorporation of 3H-thymidine into cultured cells and by flow cytometry using propidium iodide. The following results were obtained. The incorporation of 3H-thymidine into the cultured cells was increased by addition of synovial extract and the medium of cultured synovial cells. The molecular weights of active materials from the synovial extract were 13-18, 36-52, and 105-150 K daltons and those from the medium of cultured synovial cells were 13-18, 36-52 K daltons. The increase of S and G2 + M phase cells was observed flow-cytometrically 24-48 hr after addition of the synovial extract.