Production of eosinophil chemotactic factor by T-lymphocytes (ECF-L) was examined in Toxocara canis-infected mice. When spleen cells from T. canis-infected mice were cultured in serum-free RPMI1640, ECF-L production was detectable in an antigen-specific manner. The ECF-L production peaked at day 9 post-infection and then decreased. Depletion of Thy 1.2+ cells or CD8+ cells completely abrogated ECF-L production, whereas depletion of CD4+ cells did not, indicating that CD8+ T-cells are involved in the production of ECF-L. When bone marrow eosinophils obtained from T. canis-infected mice were preincubated with ECF-L, their chemotactic reactivity to parasite-derived ECFs was enhanced, whereas that of peritoneal cavity-derived eosinophils was not. Thus, ECF-L seems to be important not only as a chemoattractant but also as an activator of the chemotactic reactivity of naive eosinophils to the parasite-derived ECF in T. canis infection.