The role of eosinophils in ocular ascarid infection is studied in an animal model. Primary intravitreal infection of guinea-pig eyes with ascarid (Toxocara canis, Ascaris suum) second-stage (L2) larvae resulted in an anterior chamber exudate containing 98% or more eosinophils. Anterior chamber aspirates were cultured in RPMI media 1640 with T. canis or A. suum L2 larvae at 37 degrees C for 1-96 hours. The in vitro interaction of eosinophils with L2 larvae was studied by light, and scanning and transmission electron microscopy. Eosinophil interaction with L2 larvae in vitro was dependent upon soluble factors present in the anterior chamber aspirates from the infected eyes. Eosinophils adhered firmly to the surface of the L2 larvae, to a larval sheath, or to attached eosinophils. Eosinophils interacting with larvae displayed a range of granular matrix changes, core duplication, and reversal of the relative electron densities of the core and matrix, suggestive of eosinophil activation. An eosinophil secretory granule was seen to empty its contents onto a T. canis L2 larval surface. Electron-dense material was observed in the interphase between eosinophils and the larval cuticle or sheath. Large lipid droplets within muscle cells and ballooned-out epidermal cells were seen within larvae immediately beneath adherent eosinophils. Parasites were able to partially evade interaction with eosinophils in culture by shedding their sheaths. A similar phenomenon in vivo may allow the parasite to migrate from a site of inflammation. It is possible that a discarded sheath or membrane serves as an antigenic stimulus for a local intraocular granulomatous reaction free of parasite.