Nucleic acid amplification techniques were applied to the enzyme linked immunosorbent assay (ELISA) with an antibody-specific aptamer, R18. This novel detection system is a modification of the original immuno-polymerase chain reaction (immuno-PCR), but oligonucleotide-labeled antibodies are not required in the assay. This method is performed with the usual ELISA protocol, using an RNA aptamer for rabbit IgG instead of the conventional secondary antibody. After the assay plate was washed, quantitative reverse transcription (RT)-PCR was performed. Ribonuclease (RNase) inhibitors are not needed for this method. The detection limit of the quantitative RT-PCR is over 100 times more sensitive than the original ELISA method, even with the same sandwich-antibody combination. Only 1 mg of aptamer is sufficient for more than 10 million assays. This aptamer-based quantitative PCR successfully detected 16 attomoles (16 x 10(-18)) of vascular endothelial growth factor (VEGF). This is a cost-effective and easy method to increase the sensitivity of the rabbit antibody-based ELISA systems. The new method is referred to as immuno-aptamer PCR (iaPCR), to distinguish it from the original immuno-PCR.