Chemotactic reactivity of eosinophils obtained from the bone marrow (BM-Eo) of cyclophosphamide-treated, Toxocara canis-infected mice was compared to that of eosinophils obtained from the peritoneal cavity (PEC-Eo). BM-Eo responded well to alanyl-tetrapeptide, a synthetic eosinophil chemotactic factor of anaphylaxis (ECF-A), whereas PEC-Eo did not. Both BM-Eo and PEC-Eo showed almost equally high chemotactic reactivity to parasite-derived ECF, ECF lymphokine and complement-derived ECF. Chemotactic reactivity of BM-Eo to synthetic ECF-A was deactivated by preincubation with ECF-A. Unresponsiveness of PEC-Eo to synthetic ECF-A could be explained by chemotactic deactivation by ECF-A, because an ECF-A-like substance was detected in the ascitic fluid; this substance could deactivate the chemotactic reactivity of BM-Eo to synthetic ECF-A. From these results, BM-Eo are naive and seem to be a good indicator for eosinophilotaxis and its modulation.