We report here on the first analysis of polymerase chain reaction (PCR) products amplified from a small amount of human blood DNA by electrospray ionization mass spectrometry (ESI-MS). Adenomatous polyposis coli (APC) gene fragment of about 50 base pairs (bp) with a relative molecular mass (M(r)) of approximately 15,000 u was amplified from human blood DNA by PCR. The accurate molecular mass of the PCR products was determined with an accuracy of approximately 0.005% by ESI-MS. The amount of DNA used was only 100 ng (approximately 50 zmol; the theoretically required amount of blood is therefore less than 1 microliter for PCR). The ESI-MS measurement of the PCR products proved to be a new accurate, sensitive and fast tool for gene diagnosis.