On physical examination, the patient had multiple hyperpigmented plaques over the trunk, chest, abdomen, and back. Rubbing the lesions produced a hive-like wheal. (Positive Darier's sign). There was an intact fluid filled vesicle on the anterior chest. The biopsy was done to confirm a clinical diagnosis of urticaria pigmentosa. The Leder stain is a histochemical method for the detection of the enzyme chloracetate esterase which is found in granulocytes, including mast cells (tissue basophils)and their precursors. (See:Leder LD Am J Dermatopathol 1979 Spring;1(1):39-42 The chloroacetate esterase reaction. A useful means of histological diagnosis of hematological disorders from paraffin sections of skin.) The Leder stain is a sensitive technique that is interpreted in the context of the clinicopathologic setting and correlation with the H+E morphology. In our experience, this is more sensitive and often easier to interpret than a Giemsa stain; although, we also do sometimes use the latter method as well. Immunohistochemical methods are also now available. (see below) We do not have any personal experience with those antibodies currently.