By utilizing the colloidal gold particle technique, we localized eosinophil granule major basic protein, eosinophil cationic protein (ECP), and eosinophil-derived neurotoxin (EDN) in human nasal polyp sections by immunoelectron microscopy. Sections stained with affinity chromatography purified rabbit anti-human major basic protein, and subsequently with gold colloidal particle-goat anti-rabbit IgG, showed gold particles predominantly within granule cores, and not within other eosinophil organelles, plasma cells, mast cells, lymphocytes, or neutrophils. Sections stained with anti-ECP or anti-EDN showed gold particles concentrated over the granule matrix with fewer particles centrally. Control sections treated with preimmunization sera showed no staining of cells or organelles. These results verify the localization of major basic protein to the crystalloid core of the human eosinophil granule and show that ECP and EDN reside in the granule matrix. This technique provides a means of accurately locating the sites of major basic protein, ECP, and EDN deposition and thus of identifying eosinophil degranulation patterns in human disease.