The effects of Toxocara canis infection on hemopoietic stem cells and hemopoietic factors were examined in mice. Severe eosinophilia was observed with a peak 14 days after infection. When the numbers of hemopoietic stem cells in peripheral blood, spleen, and bone marrow were examined by spleen colony assay (CFU-S), those in peripheral blood and spleen increased in parallel with peripheral blood eosinophilia. On the other hand, CFU-S in bone marrow did not alter significantly throughout the course of infection. Interleukin (IL)-3, which is known as multi-colony-stimulating factor and is involved in the growth/differentiation of various blood cells including stem cells, was produced by spleen cells of infected mice. The time course study showed that concanavalin A stimulated IL-3 production peaked on day 7 after infection, whereas that with excretory secretory antigen peaked on day 14. Even without stimulation, spleen cells obtained on day 21 after infection produced IL-3 spontaneously. IL-5, which is known to have eosinophil differentiation factor activity, was also produced by spleen cells obtained on day 13 after infection. These results suggest that in response to increased demand for eosinophils, hemopoietic stem cells migrate into various extramedullar hemopoietic organs where they grow/differentiate into mature eosinophils, depending on the hemopoietic factors.