In an attempt to identify autoantigens of synovium in rheumatoid arthritis (RA), we constructed lambda phage expression cDNA libraries from synovium and screened them by IgG purified from synovial fluids, both of which were derived from RA patients. As a result of this unique combination of the libraries and probes, we cloned follistatin-related protein (FRP) as a novel autoantigen in systemic rheumatic diseases. FRP is a secreted protein containing a similar amino acid sequence to follistatin, an inhibitor of activin. FRP was first cloned as a transforming growth factor-beta1-inducible protein (called TSC-36) from a mouse osteoblastic cell line and was suggested to have some roles in the negative regulation of cellular growth. Immunoblotting analyses detected synovial fluid and serum anti-FRP antibodies of IgG class more frequently in RA than any other systemic rheumatic diseases and controls. Synovial fluid anti-FRP antibodies appeared in 44% of RA (n = 18) and none of osteoarthritis (OA) (n = 15) patients. Serum antibodies were detected in 30% of RA (n = 67), 17% of systemic sclerosis (n = 18), 10% of systemic lupus erythematosus (n = 51) and Sjögren's syndrome (n = 10), and none of polymyositis/dermatomyositis (n = 13) patients and healthy subjects (n = 30). These antibodies recognized an EC domain, an extracellular Ca2+ binding module. In anti-FRP antibody-positive RA patients, serum C-reactive protein level and erythrocyte sedimentation rate were more elevated than negative patients (P < 0.05 and P < 0.01, respectively). FRP gene expression was higher in RA than OA synovium (P < 0.05). However, there was no difference between these groups in the amount of synovial FRP, suggesting its elevated turnover in RA. As follistatin inhibits activin, FRP might inhibit some growth factor-like molecule. Detection of anti-FRP antibodies, possibly having disease-promoting effects as the blocking antibodies, could be one of the markers for clinical evaluation of systemic rheumatic diseases.