The South American fern Polypodium decumanum, traditional name calaguala, has documented clinical use in oral treatment of skin disorders, including psoriasis. The inflammatory mediator platelet-activating factor (PAF), has been implicated in the pathogenesis of psoriasis. A constituent of a calaguala extract has been shown to have inhibitory activity in a PAF-induced exocytosis model in human neutrophils. The compound was identified as the sulphoquinovosyl diacylglycerol 1,2-di-O-palmitoyl-3-O-(6-sulpho-alpha-D-quinovopyranosyl)-glycero l by spectroscopic means. When subsequently studied in an in-vitro model for [3H]PAF binding in neutrophils from man the compound caused dose-dependent displacement of [3H]PAF from its receptor with an IC50 value of 2 microM. It is suggested that the compound acts through PAF receptor antagonism in intact human neutrophils.