The properties of the hepatitis-A-virus and the hepatitis-B-virus are described. The authors refer to the fact that despite the far-reaching development, particularly in the field of the hepatitis-B-infection up to a usable active immunoprophylaxis many questions concerning the natural history, pathogenesis and epidemiology are still unclarified. The development of demonstration methods of the 3rd generation and their use in the diagnostics of hepatitis rendered possible the gain of new realizations. The authors contributed to the introduction of these methods essentially by own developments, as for instance of ELISA for the demonstration of HBeAg and anti-HBe. The solution of some still open questions can be expected only by the successful establishment of a suitable virus-host-cell system.