Significant chemotactic activity for eosinophils was detected in soluble egg antigen (SEA) preparations of both Schistosoma japonicum and S. mansoni in dose-dependent fashion. The activity of S. japonicum SEA was higher than that of S. mansoni SEA. Gel filtration on Sephadex G-150 showed that S. japonicum SEA was composed of two groups of eosinophil chemotactic factors (ECFs), one of high molecular weight (JEE-H) and the other of low molecular weight (JEE-L). S. mansoni SEA showed ECF composition similar to that of S. japonicum SEA. JEE-H was stable on heating (100°C, 60 min) and resistant to pronase digestion, but was sensitive to periodate oxidation. JEE-L was also stable on heating and resistant to pronase and carboxypeptidase A digestions. These properties of the ECFs were also held in common with those of S. mansoni SEA. JEE-L was extractable by toluene, indicating a hydrophobic nature. These results suggest that schistosome eggs themselves contain ECFs, and that the composition of S. mansoni and S. japonicum SEA-derived ECFs is essentially the same. However, they differ from the other ECFs which have already been described in schistosome infections.