Leukocytes from subjects allergic to Dermatophagoides pteronyssinus were incubated for 20 min with a solution of D. pteronyssinus extracts. Histamine release was measured at 0, 3, 10 and 20 min. Simultaneously, samples were treated for electron microscopy in such a way as to correlate histamine release and the morphological aspects of basophil leukocytes. The principal features accompanying histamine release were: a progressive activation of the cytoplasmic membrane which showed long processes, densification of the mitochondria, fusion of granulations, progressive dissolution and exocytosis of the contents of the granulations, short segments of rough endoplasmic reticulum, active Golgi apparatus, and thin membrane-bound granules suggesting resynthesis of mediators.