Treatment of amino-group-containing antigens with adenosine-5′-trimetaphosphate results in their chemical modification by -pppA residues. An immunoanalytical system is proposed based upon competition of these ATP-labelled antigens with those of the sample for immobilized antibodies. Mild acidic treatment of complexes of ATP-labelled antigens with immobilized antibodies results in quantitative liberation of intact ATP. The latter may be determined by the ultrosensitive bioluminescent techniques based upon emission of light with firefly luciferase. The validity of the system has been studied with two clinically important antigens, thyroxine and myoglobin.
