Studies were undertaken in order to determine if Anisakis larva itself has the ability to attract eosinophils. Extracts from Anisakis larvae were examined for both neutrophil and eosinophil chemotactic activities with both in vivo and in vitro assay systems. When the soluble extract was injected intradermally into normal guinea pigs, a profound accumulation of eosinophils was observed at the injection site. The cells started to accumulate at the site within 1 hr and the number of eosinophils at the site reached its peak at 8 hr after the injection of the extract. Such eosinophilic accumulation was enhanced in a dose-response fashion over the range of 0.5 to 50 micrograms protein. The strong chemotatic activity of the Anisakis extract for eosinophils was confirmed when in vitro chemotaxis assays were performed with Boyden chemotatic chambers. Interestingly, no chemotactic activity for neutrophils was found at those concentrations of the extract with which the eosinophil effect was observed. These results indicate that the factor described here, in addition to the various known immunologic factors, may play an important role in the development of eosinophilia in anisakiasis.