Australian antigen (Au Ag) was first described by Blumberg in the serum of an Australian aborigen. Its main clinical significance is based on the fact of its association with the presence of hepatitis B (Okochi, Nurakami, 1968; Prince, 1968) (9). Almeida (1969) outlines the intervention of immune complexes on the production of the referred illness. Later works show the importance of the type of immunologic response created by Au Ag (2). Alpert et al. (1) (1970) state the possibility that immune complexes developed by Au Ag and anti Au Ag antibodies may produce in patients with hepatitis B other allied manifestations including arthritis. On the basis of similar immunopathological findings (illnesses including immunocomplexes on their production), Au Ag investigation in rheumatoid arthritis (RA) has been the object of several works, whose results reveal certain discrepancies. We have investigated the presence of Au Ag on 70 patients whose diagnostic was RA "classical (according to the judgement of the American Rheumatism Association). We used immunoelectrophoresis (counter immunoelectrophoresis-CIEP-) and radioimmunoassay (RIA) techniques. We prepared individual records including clinical, epidemic, biochemical and personal data. Au Ag investigation both through different techniques gave negative results in 70 cases, of which only four referred a possible previous hepatitis B clinic history, not showing during the investigation suggestive clinical data of hepatitis or biochemical alterations. Panus et al. (1970) (5), Desche-Labarte et al. (1972), Trempo (1972), Burrel (1973) (6), Serré et al. (1973) (8), obtained negative results in the matter, with similar or lower casuistics than ours. Notwithstanding Roqués et al. (1975) (7), in 300 cases of RA showed the presence of Au Ag in a significant number of cases (5%) as compared to Au Ag incidence in the French population (estimated in about 0.22%) (6). However, Roqués did not find in their Au Ag positive cases any evidence which enables to think that this antigen has played an important role on the origin and development of the rheumatic illness. These important discrepancies may be due to different reasons: a) Use of different techniques for Au Ag detection. b) The transitory character of the presence of Au Ag in the serum. c) Important differences in the antigen geographic distribution. d) Previous hepatitis B. There are som other factors which have attained importance recently, as a consequence of the broad epidemiological studies carried out. So, Au Ag increased incidence has been found--statistically significant--on groups of persons who have been submitted to several surgical interventions and/or have needed repeated hospitalizations or reiterated blood transfusions (6). These three conditions--especially the first two--are accomplished in an illness with chronic and invalidating characteristics as RA...