The mechanism of action of 5-ASA and its derivatives remains obscure. The most likely mechanisms would appear to be an interrelationship between local production of eicosanoids and reactive oxygen metabolites, in conjunction with activated neutrophils, though the nature of the initial triggering factor/s remains to be determined. The possibility that bacterial-derived chemotactic peptides, such as FMLP, might be a triggering factor is an attractive possibility, made possible by an underlying defect in mucosal permeability.