Abstract.Quantitation of circulating Australia antigen (Au‐antigen) was performed by electrophoresis in antibody‐containing gel, using a modification of the method of Laurell [7], on consecutive serum specimens from 19 patients with uncomplicated acute viral hepatitis associated with transient Au‐antigenaemia, and from 9 patients with persistence of Au‐antigen and histological progression from acute to chronic hepatitis. None of the patients received corticosteroids or other immunosuppressive drugs. In all cases in the group of patients with uncomplicated acute hepatitis, an exponential fall in the Au‐antigen concentration was observed. In the group of patients with persistent antigenaemia, two phases of the disease were observed. Initially, these patients showed great variations in both the Au‐antigen concentration and in the serum glutamic oxalacetic transaminase activity (SGOT). An inverse relationship between the Au‐antigen concentration and the transaminase values was observed. The next phase, 1–2 years after onset of the disease, was characterized by low values of the Au‐antigen concentration and