The kinetics of eosinophil growth and/or survival stimulating factor (Eo-stimulating factor) production in Angiostrongylus cantonensis-infected rats was assessed by in vitro marrow cultures. When lymphocytes, obtained from cervical, mediastinal or mesenteric lymph nodes of infected WKAH rats, were cultured with A. cantonensis antigens, Eo-stimulating factor activity was detected in the conditioned media obtained only from mediastinal lymph node cells at and after 35 days p.i. Eo-stimulating factor activity in the conditioned media was not detected before or at 20 days p.i. The kinetics of the factor was, therefore, almost identical to those of eosinophilia in bone marrow and peripheral blood. This Eo-stimulating factor activity was inhibited (maximum inhibition = 73%) by anti-mouse IL-5 monoclonal antibody in a dose-dependent fashion, indicating that the factor would be, at least in part, identical to IL-5. Similar Eo-stimulating factors were also detected in the conditioned media which were obtained by stimulating spleen cells from Wistar rats at 35-45 days p.i. with Con A. These data suggest that lymphocytes in the mediastinal lymph nodes, local lymph nodes near the adult worm habitat, could play an essential role in induction of eosinophilia in A. cantonensis-infected rats.