A very sensitive method for detection of antigens. The antigen is immobilised in wells of a microtitre plate, exposed to its specific monoclonal antibody, then to a chimaeric protein that consists of a protein A moiety, which will bind tightly to the immunoglobulin G antibody, and a streptavidin moiety, which will bind to a biotinylated DNA fragment. The fragment is amplified by PCR, separated by gel electrophoresis and stained with ethidium bromide. The chimaeric protein, biotinylated DNA fragment and PCR primers may be used to assay any antigen-antibody pair. The system is sensitive to as few as 580 antigen molecules (10^-21 mol).