Platelet-activating factor (PAF) is a biologically active phospholipid synthesized by a variety of cells upon stimulation. PAF is converted to the biologically inactive lyso-PAF by the enzyme PAF acetylhydrolase (PAF-AH). Plasma PAF-AH is highly selective for phospholipids with very short acyl groups at the sn-2 position and is associated with lipoproteins. The Cayman Chemical PAF acetylhydrolase assay kit provides an accurate and convenient method for measurement of PAF-AH activity. The assay uses 2-thio PAF which serves as a substrate for PAF-AH. Upon hydrolysis of the acetyl thioester bond at the sn-2 position by PAF-AH, free thiols are detected using 5,5"-dithio-bis(2-nitrobenzoic acid) (DTNB, Ellman"s reagent). The dynamic range of the assay is only limited by the accuracy of the absorbance measurement. Most plate readers are linear to an absorbance of 1.2. The detection range of the assay is from 0.02 to 0.2 µmol/min/ml of PAF-AH activity which is equivalent to an absorbance increase of 0.01 to 0.1 per minute. Each kit contains assay buffer, DTNB, 2-thio PAF (substrate), human PAF-AH standard, a 96 well plate, and complete instructions.