Urticaria pigmentosa, a cutaneous form of mastocytosis, is associated with the excretion of above normal amounts of histamine in the urine.1,2 It is estimated that approximately one-third of patients with this condition experience spontaneous attacks of flushing of the skin,1 and it is thought that this flushing is due to release of histamine from the mast cells. If this is true, the administration of a histamine liberator to these patients should produce flushing, and repeated administration of such an agent might maintain the mast cells in a state of histamine depletion, thus preventing further flushing attacks and whealing of the local lesions. Codeine3 and polymyxin B4,5 are 2 potent histamine liberators, the latter being particularly specific for histamine. This paper reports observations on the effects of these 2 agents administered to 5 patients with mastocytosis. Total urinary histamine excretion was measured in one case. In this