The ultrasensitive detection of 2,4,6-trinitrotoluene (TNT) was accomplished on the basis of sandwich-type TNT immunoassay combined with electrogenerated chemiluminescence (ECL) technology. Biotinylated anti-TNT species were attached to the surface of 1-microm diameter streptavidin-coated magnetic beads (MB) and 10-microm diameter avidin-coated polystyrene microspheres/beads (PSB) pre-loaded with ECL labels ( approximately 7 billion hydrophobic ruthenium(II) tris(2,2'-bipyridine) (Ru(II)) molecules per bead) to form anti-TNT<-->MB and anti-TNT<-->PSB(Ru(II)) conjugates, respectively. Sandwich-type PSB(Ru(II))<-->anti-TNT<TNT>anti-TNT<-->MB aggregates were formed when PSB(Ru(II))<-->anti-TNT was mixed with anti-TNT<-->MB conjugates in the presence of analyte TNT and 2.0% bovine serum albumin blocking agent. The newly formed aggregates were magnetically separated from the aqueous reaction media and dissolved in acetonitrile containing 0.10 M tri-n-propylamine ECL coreactant-0.055 M trifluoroacetic acid-0.10 M tetrabutylammonium tetrafluoroborate electrolyte. ECL as well as cyclic voltammetric measurements were carried out with a potential scan from 0 to 2.8 V vs Ag/Ag(+), and the integrated ECL intensity was found to be linearly proportional to the analyte TNT concentration over the range of 0.10-1000 ppt (pg mL(-1)). The limit of detection (<or=0.10+/-0.01 ppb) is about 600x lower as compared with the most sensitive TNT detection method in the literature, and the absolute detection limit in mass ( approximately 0.1 pg) is only approximately 0.5% of that from mass spectroscopy. The approach coupled with the standard addition method was applied to measure the TNT contaminations in soil and creek water samples collected from a military training base.