Eosinophil chemotactic factor (ECF) was detected in the cell-free supernatant of the culture of spleen cells obtained from Schistosoma japonicum-infected mice. ECF production by spleen cells was dependent on both the amount of soluble egg antigen (SEA) added and the number of cells in the culture. When sufficient amount of SEA was added to the culture, ECF production was detectable by 6 h after incubation, reached a plateau by 24 h and then rapidly decreased. ECF was produced by nonadherent, Thy 1.2-positive cells, indicating that it is a lymphokine. After Sephadex G-75 gel chromatography, estimated molecular weight of this ECF was 15,000–20,000 daltons. When eosinophils were preincubated with this ECF lymphokine, their chemotactic reactivity to S.japonicum egg-derived ECF was significantly enhanced, although this pretreatment caused neither the deactivation of chemotactic reactivity to homologous ECF nor the enhancement of the random migration of eosinophils. Thus, this ECF lymphokine seems to be important not only as a chemoattractant but also as an activator of eosinophilotaxis around deposited eggs in schistosomiasis japonica.