Polymyxin B is known to bring about degranulation of tissue mast cells and release of histamine in the rat and hamster.3-5 The following experiment was carried out on a patient with generalized mastocytosis as an attempt to change the clinical condition which had turned out to be fatal and resistant to any applied therapy. The history of the patient has been published in the A.M.A. Archives of Dermatology, 1960.1 During 5 years the man, aged 34, had developed a generalized mastocytosis involving skin (urticaria pigmentosa), liver, spleen, lymph glands, lungs, bones, bone marrow, and blood. The condition was leukemoid with the unique characteristic of pronounced blood basophilia. After a complete failure of intense treatment with prednisone and an antimitotic drug (Mitostan), treatment with polymyxin B (Pfizer), 50 mg. twice daily, was started. Four days after the first injection and through all the period of treatment it was not