This direct competitive ELISA is designed to measure the level of S-adenosylmethionine (SAM) in the sample. SAM conjugated with macromolecule is immobilised on the micro-titer plate. Standards and samples are pipetted into the wells, and then the Anti-SAM Antibody (HRP) is added. The free SAM molecule in samples or standards competes with the immobilised SAM on the micro-titer plate surface for binding sites of the antibody. After discarding the mixed solution and washing each well, TMB substrate solution is added. The substrate solution turns blue under the effect of HRP, and changes into yellow once the stop solution (acid) is added. The colour develops in inverse proportion to the amount of SAM in the sample (or standards). The optical density of the remaining solution (OD450) is measured at 450nm using micro-plate spectrophotometer. The level of SAM in samples can be calculated through standard curve generated with standards.