Bucillamine (N-(2-mercapto-2-methylpropionyl)-L-cysteine), an antirheumatic drug, is used clinically in Japan and in Korea primarily to treat patients with rheumatoid arthritis (RA). Bucillamine has structural similarities to D-penicillamine, but contains two free sulfhydryl groups, resulting in molecular and therapeutic effects significantly different from D-penicillamine. Bucillamine appears to be more effective in treating RA than D-penicillamine. Bucillamine has been characterized as an immunomodulatory agent with suppressive effects on various B and T cell functions. In clinical use for RA, bucillamine attenuates rheumatoid factor titers, erythrocyte sedimentation rates, and C-reactive protein levels. These actions suggest bucillamine has anti-inflammatory and immunomodulatory effects in cases of RA. However, the details of bucillamine`s anti-rheumatic mechanism with respect to its effects in RA therapy remain unknown. Our review discuses bucillamine`s antirheumatic mechanism, based primarily on previously published reports. It is our hope that bucillamine will eventually benefit a broader range of RA patients.