The ability to detect antigens immunologically is limited by the affinity of the antibodies and the amount of antigens. We have now succeeded in creating a modular, facile amplification system, termed fluorescent amplification catalyzed by T7 polymerase technique (FACTT). Such a system can detect protein targets specifically at subfemtomolar levels (∼0.08 fM). We describe here the detection of Her2 (also known as Neu) from rodent and human sera. FACTT is adaptable to high-throughput screening and automation and provides a practical method to enhance current ELISAs in medical practice.