The physicochemical nature of eosinophil chemotactic factor (ECF) was examined by using different maturation stages of eosinophils, namely eosinophils obtained from bone marrow (BM-Eo) and those obtained from peritoneal cavity (PEC-Eo), as the indicator of in vitro chemotaxis. As the ECF source, the conditioned medium of the culture of spleen cells obtained from Schistosoma japonicum-infectedmice was used. When the conditioned medium was chromatographed by Sephadex G75, apparent molecular weight of ECF activity against BM-Eo was 15,000, whereas that against PEC-Eo was 30,000. Similar results were obtained when the conditioned medium was chromatographed by high-pressure liquid chromatography using SW3000 column. After NaCl-linear gradient elution of DE52 anion-exchange chromatography of the conditioned medium, ECF activity against PEC-Eo was detected at the 0.1 M NaCl region of the gradient, whereas that against BM-Eo was detected at around the 0.14 M NaCl region. After isoelectric focusing, however, ECF activity against BM-Eo and PEC-Eo was detected at the same position, approximately pH 3.6. The results of surface phenotype analysis show that the cells responsible for the production of two types of ECF lymphokines are both Thy-1.2+, Lyt-1.2––, Lyt-2.2+. When ECF activity in the conditioned medium of antigen-specific T cell clones was examined, 6 out of 7 clones produce ECF lymphokine relatively selective to PEC-Eo, whereas one clone produce ECF lymphokine relatively selective to BM-Eo. These results suggest that physicochemically different ECF lymphokines selective to different maturation stages of eosinophils are involved in the mediation of eosinophilia.