The aim of this study was to construct and purify a novel interleukin-1 receptor antagonist (IL-1ra)-interleukin-10 (IL-10) fusion protein and determine its biological function and anti-inflammatory effects. The isolated cDNAs of two inhibitory cytokines (IL-1ra, IL-10) were used to construct a cDNA for the IL-1ra-IL-10 fusion protein. The expressed recombinant cytokines and fusion product were purified and their biological properties analysed. The anti-IL-1 effect was evaluated by using a thymocyte-proliferation assay, and the IL-10 effect was investigated by the inhibition of interferon-gamma (IFN-gamma) production from splenocytes. The clinical response and histological analyses were studied in an adjuvant arthritic rat model. The fusion protein was 38 000 molecular weight in size. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting demonstrated that the purified protein was recognized by both IL-1ra and IL-10 antibodies. The fusion protein significantly inhibited IL-1-mediated thymocyte proliferation and concanavalin A (ConA)-primed IFN-gamma production from splenocytes. The fusion protein also suppressed joint swelling (paw circumference reduced from 5.0 +/- 0.2 to 4.1 +/- 0.1 cm; paw thickness approximately 2 mm in difference) and synovial inflammation in adjuvant arthritis of rats. Our investigations indicate that this fusion protein effectively suppresses inflammatory arthritis and may initiate a trend for future clinical application to target multiple molecules at the same time.