A method of determining the concentration of a virus or antigen thereof in a sample comprises the steps of: providing a sensor surface having immobilized thereto a virus antigen or a virus antigen analogue, mixing the sample with a known amount of antibody to the virus antigen to obtain a predetermined concentration of antibody to the antigen in the sample mixture, contacting the sample mixture with the sensor surface to bind free antibody in the mixture to the sensor surface, measuring the response of the sensor surface to the binding of free antibody, and determining the concentration of the virus or antigen in the sample from a calibration curve prepared by measuring the responses obtained for mixtures containing the predetermined concentration of antibody and different concentrations of virus.