The AA. carried out the research of Australia Antigen by radioimmunoassay (RIA) as well as with counter-electrophoresis technique (CE) on 142 patients with acute viral hepatitis. For comparison the RIA determination of the Australia Antigen was also performed on 26 blood donors, already established Au positive by a preliminary screening with CE method. On the 142 patients examined, the RIA permitted the detection of 19 as Au positive above the number evidenced by the less-sensitive CE technique. The Au antigen levels, determined quantitatively in conventional units (c.u./0,1 ml of serum), continually decreased with the progress of the disease toward recovery. Yet, in 23 cases (44,3%), the antigen was still present in the serum after normalization of transaminases. The AA. call attention to the importance the Au positive hepatitis convalescents and Au positive blood donors could have in the transmission of the disease. The antigen levels of blood donors, when determined with RIA, were found comparable to those observed in the acute phase of viral hepatitis.