Distribution of IL-3 producing T cells, STIL-3, in splenectomized mice after intravenous injection was studied in order to determine the site of IL-3 production in the splenectomized mice. Hematological features of STIL-3-inoculated mice after splenectomy were also studied. Splenectomy prior to inoculation of STIL-3 cells resulted in a slight increase in the level of IL-3 activity in most lymphohemopoietic tissues, but resulted in a slight decrease in the level of IL-3 activity in the plasma. The splenectomy also abrogated granulocytosis which was evident in sham-operated host mice. Hepatomegaly and a number of granulocytic foci in the liver were observed in STIL-3 inoculated mice, but this was again abrogated completely by the splenectomy. These results suggest that the spleen is the major site of granulopoiesis in response to the stimulus with IL-3, and also that the spleen is essential for the migration or metastasis of STIL-3 cells to the liver.