The authors have studied the incidence of hepatitis associated antigen (HAA) and the homologous antibody in sera of patients with rheumatoid arthritis on the basis of (1) arthritis sometimes associated with viral hepatitis, (2) the possible infectious etiology of rheumatoid arthritis, and (3) observation on the possible pathogenetic role of HAA in some cases of polyarteritis nodosa. The presence of HAA and antibody titer gave constantly negative results in all subjects examined with the exception of one case which showed no signs of serological or histological hepatic involvement. On the basis of the results obtained, the negligible role of HAA in the etiopathogenesis of rheumatoid arthritis is underlined. However, the authors emphasize as suggestive the hypothesis that the characteristic histopathological alterations of rheumatoid arthritis may be mediated by an immunological reaction toward an infectious agent other than HAA, but operating through mechanisms similar to those of HAA in polyarteritis nodosa.