We examined eosinophil degranulation in tissues from patients infected with Baylisascaris procyonis as shown by the extracellular deposition of granule major basic protein (MBP). We utilized immunofluorescence to localize MBP in eosinophils and at sites of degranulation to study specimens from 2 fatal cases of B. procyonis infection. Large numbers of intact eosinophils were present in the brain around blood vessels and necrotic migration tracks and in mesenteric granulomata. Extensive extracellular MBP deposition was present in the necrotic migration tracks in the brain and around larvae in the mesenteric granulomata in association with the radiating eosinophilic deposits characteristic of the “Splendore-Hoeppli phenomenon.” The Splendore-Hoeppli deposits consist in part of eosinophil granule MBP. Release of the cytotoxic MBP in response to invading larvae may cause tissue damage. Central nervous system tissue damage by cytotoxic eosinophil granule proteins may contribute to the neurologic symptoms of B. procyonis infection.