The efficiency of a vaccine of inactivated virus is influenced to a great deal by the mass of antigen per dose of application. Therefore it is essential to known the concentration of the antigen. We tested a physical method for quantification of the RHD-Virus. It implies a centrifugation of the prepurified, if necessary, preconcentrated infectious or inactivated virus in a gentle sucrose gradient. It is followed by analysis in a sensitive UV flow-through photometer with a computer calculated virus mass. The extinction coefficient (optical density) of the virus (175S-component) at 254 nm is 3.9 cm2/mg. The haemagglutination test and the ELISA were used to prove the virus specificity of the optical peak and partial for comparing them with the method mentioned above.