An indirect immunofluorescence assay using formalin‐fixed paraffin‐embedded skin was performed on six biopsies from four patients with eosinophilic cellulitis (Wells' syndrome) to determine the extracellular localization of eosinophil granule major basic protein (MBP). Serial sections from each biopsy were treated with either affinity chromatography‐purified antihuman‐ MBP or staphylococcal protein A purified rabbit IgG (control material). There was striking extracellular fluorescence localized to flame figures, and intracellular staining of eosinophils in all sections treated with anti‐MBP as compared with controls. The pattern of MBP extracellular staining corresponded to the configuration of each flame figure (as verified by counterstain of the same section with haematoxylin and eosin). These findings show that MBP can be used as a marker for determining eosinophil degranulation and, because MBP is localized to flame figures, they suggest that MBP may play a pathogenic role in Wells' syndrome.