The growth of adherent synovial cells passaged once was studied in response to human recombinant interleukin 1 (hr IL-1) beta. Human synovial cell cultures were established from tissues obtained during therapeutic joint surgery for patients with rheumatoid arthritis (rheumatoid synovial cells, RSC) or non inflammatory rheumatic diseases (non rheumatoid synovial cells, NRSC). The effect of IL-1 beta (0.1 to 10 ng/ml) on the time course of proliferation showed that values for DNA synthesis and cell numbers in RSC cultures were higher than in NRSC cultures. Similarly, untreated control RSC cultures grew more quickly than NRSC. These results demonstrate that RSC, which are continuously stimulated by IL-1 beta produced in the rheumatoid pannus in vivo, have a higher capacity for proliferation than NRSC but are less responsive to IL-1 beta. A dose-response curve of proliferation was established 72 hrs. after the addition of IL-1 to the medium. The stimulating effect of IL-1 beta (0.001 to 10 ng/ml) was dose-dependent in both RSC and NRSC and reached a plateau at 10 ng/ml; the response of NRSC was stronger than that of RCS.