The eosinophil has long been associated with diseases of acute hypersensitivity and with parasite infections, but its exact role in the pathogenesis of these conditions remains uncertain. Characterization of factors associated with migration of eosinophils into tissues has helped to elucidate eosinophil function. Eosinophil chemotactic factors associated with acute hypersensitivity reactions include the eosinophil chemotactic factors of anaphylaxis, histamine, and arachidonic acid metabolites, all of which are released from mast cells, and the lymphokine eosinophil stimulation promoter (ESP). Eosinophilotaxins associated with parasitic diseases include the lymphokine ESP and the low molecular weight factor ECF-G, both associated with schistosome infection in mice. In addition, in several parasite infections parasite-derived protein eosinophil chemotactic factors have been identified and characterized. The proteins associated with Ascaris, Anisakis, and Schistosoma infections appear to be distinct from one another. We have recently partially characterized a protein from Taenia taeniaeformis larvae which has marked chemotactic activity for eosinophils. In addition we have demonstrated eosinophil chemotactic activity associated with metabolism of arachidonic acid by T. taeniaeformis metacestodes. The results of studies in taeniasis and other parasite infections, therefore, indicate that parasite-derived factors may directly influence migration of eosinophils.