Abstract Particulate fractions were prepared from the sera of three patients with clinical and histological evidence of hepatitis, from a pooled serum specimen prepared from ten patients with hepatitis, from a patient with healing hepatitis, and from one healthy volunteer. The pelleted material concentrated by ultracentrifugation was found to contain Australia (Au) antigen by immunodiffusion and complement-fixation tests in the three patients with hepatitis, and in the pooled specimen. These four pellets showed many particles on electron-microscopic examination. The pellets were examined for D.N.A. and R.N.A. synthesising activities in enzyme assays. No R.N.A. synthesising activity was observed. However, specimens strongly positive for Au antigen had weak endogenous D.N.A.-synthesising activity, as measured by the incorporation of 3H-T.M.P. into a D.N.A.-like product. This endogenous synthesising activity could be abolished by predigesting the Au antigen sample with R.N.ase. The reaction was much magnified by using dAT as a template. The kinetics of the reactions were slow, and the rate of incorporation of 3H-T.M.P. decreased after 3 hours' incubation. These results indicate the presence of a D.N.A. polymerase in concentrated preparations containing Au antigen.