Eosinophils isolated from peritoneal exudates were labeled with indium-111-oxine and injected intravenously into sensitized mice. They became localized at sites of inflammation produced by intradermal injections of schistosomal antigen or Toxocara canis larvae, whereas labeled neutrophils did not. Intense uptake of eosinophils by normal spleen, liver, and bone marrow was noted, with tracer distribution effectively complete by 5 hours after injection. Indium-111-eosinophil studies appear to be quite sensitive to parasitic inflammatory reactions; in contrast, nonspecific inflammation such as that induced by turpentine causes localization of eosinophils, but to a lesser extent. This technique may be useful in the study of parasitic and allergic disease.