5-Aminosalicylic acid seems to be the active moiety of sulfasalazine in the treatment of chronic inflammatory bowel disease. Even if the precise mode of action is obscure, it is assumed that two of the main mechanisms are inhibitory effects on the lipoxygenation of arachidonic acid and interaction with free radicals. As 4-aminosalicylic acid has been claimed to be beneficial in the topical treatment of ulcerative colitis, it was tested whether this drug possesses any influence on the 5-lipoxygenase activity in human neutrophils in vitro or whether it acts as a radical scavenger. The change of the amino residue from carbon-5 to carbon-4 abolished the effect in the two systems tested. The reported clinical observations on 4-aminosalicylic acid in the treatment of chronic inflammatory bowel disease remain to be confirmed and cannot be explained by interference with arachidonic acid metabolism or free oxygen radicals.