In the previous study, we found an eosinophil chemotactic factor of the parasite (ECF-P) in extracts from the body cavity fluid of the Ascaris species. In this study, the physiochemical characteristics of ECF-P were determined in order to elucidate the relationship between ECF-P and Ascaris antigens. ECF-P was active in both in vivo and in vitro chemotactic assay systems and was found to be heat labile, nondialyzable, and stable to lyophilization. The estimated m.w. was approximately 30,000 daltons. The isoelectric point of ECF-P was 8.5. It was quite different from the Ascaris antigen in the same extract, or various Ascaris allergens that had been reported previously by other researchers. Furthermore, ECF-P did not show any activity in eliciting a guinea pig passive cutaneous anaphylaxis (PCA) reaction. From these experimental results, we concluded that ECF-P is not identical to Ascaris antigens. In addition, we found that the neutrophil chemotactic factor of the parasite (NCF-P), with at least two components, is present in the Ascaris extract, and is also heat labile and resistant to dialysis and lyophilization, but separable from coexistent ECF-P by reason of their different physiochemical properties.