Abstract Many animal models have been developed for a study of rheumatoid arthritis (RA). However, RA animal models are not always similar to RA patients in their response to antirheumatic drugs. Recently, humanized monoclonal antibody (mAb) has been developed for the treatment of RA, but at present there is no animal model on which to screen this mAb therapy because of problems with cross-reactivity. We therefore considered the development of a novel animal model for the screening of antirheumatic drugs using the severe combined immunodeficiency (SCID) mouse in order to prevent the rejection of human transplant cells. Following subcutaneous implantation of synovial tissue in the SCID mouse, all target cells within the SCID-HuRAg mouse were of human origin, having migrated from the implanted tissue. Moreover, human interlukin-6 and rheumatoid factor were detected in this mouse serum. We therefore propose that this SCID-HuRAg mouse is a novel, useful animal model for the study and development of new drugs for RA patients. This novel RA animal model is reviewed in this chapter.