Method for assaying antigens

A method for assaying an antigen adsorbed on a solid support is provided. The method comprises: a) contacting a suspension of the antigen adsorbed on the solid support with a solution or suspension of an optionally detectably-labelled primary antibody to the antigen to form a solid-supported antigen-antibody complex; b) where the primary antibody is detectably labelled, optionally measuring the detectable label thereby to determine the quantity of the antigen; or c) contacting the solid-supported antigen-antibody complex with a detectably-labelled probe for the solid-supported antigen-antibody complex; and d) measuring the detectable label on the probe, thereby to determine the quantity of antigen. The antigen is preferably a component of a sub-unit vaccine, and most preferably anthrax protective antigen.