Tioxamast is an inhibitor of the release of histamine from the mast cell. Tioxamast decreased IgE- and IgG-dependent passive cutaneous anaphylaxis in rats at doses having no effect on histamine- and serotonin-induced capillary permeability. It has powerful oral anti-allergic properties in immediate hypersensitivity models in rats. Tioxamast was inactive against reverse passive Arthus pleurisy induced in rats by rabbit anti-bovine-albumin serum. Tioxamast inhibits the synthesis in vitro of leukotriene B4 (LTB4) by peritoneal neutrophils from rats stimulated by A23187). At higher tioxamast concentrations, metabolites of the cyclo-oxygenase pathway are inhibited at concentrations of the same order of magnitude as those that inhibit Naja naja phospholipase A2. Tioxamast also reduces the production of free radicals by leukocytes from the pleural cavity of rats which had phagocytosed opsonized zymosan. The anti-inflammatory and antiallergic effect of tioxamast makes it a potentially useful drug in the treatment of allergies in humans.