The effect of interleukin 1 (IL-1)-like factor(s), produced by cells isolated from the synovial fluids of rheumatoid arthritis (RA) patients, on an in vitro murine model of spontaneous autoimmunity, i.e., the development of plaque-forming cells (PFC) to bromelain-treated mouse red blood cells (Br-MRBC) in mouse peritoneal cell (PC) cultures, has been investigated. It has been found that IL-1-containing culture supernatants from cells isolated from joint fluids of RA patients, as well as recombinant IL-1, determine a marked increase in anti-Br-MRBC PFC development. Moreover, factor(s) of 10-20 KD molecular weight, with IL-1-like biological activity, capable of increasing the anti-Br-MRBC PFC development in mouse PC cultures, have been demonstrated in joint fluids from RA patients. The finding that synovial inflammatory cells produce factors that activate autoreactive B cells further supports the role of autoimmunity in the pathogenesis of rheumatoid arthritis, as self-perpetuing disorder.