Platelet activating factor was isolated from the lesional scale of psoriatic patients using the method described by Bligh and Dyer (8). The extract was subjected to thin layer chromatography, and the region of the plate co-migrating with platelet activating factor removed. A portion of each sample was assayed for aggregating activity using washed guinea-pig platelets and the remainder treated with phospholipase C, derivatised, and subjected to reversed phase high performance liquid chromatography. Fractions were analysed for platelet activating factor using capillary gas chromatography-mass spectrometry. Nanogram quantities of platelet activating factor were recovered from 100 mg scale and both the C16 and C18 alkyl substituents were present in the ratio 3:1, C16:C18.