IL-9 is a potent, T cell-derived, T cell growth factor which can also enhance mast cell activity and IL-3- or IL-4- dependent proliferation of bone marrow-derived mast cells. IL-9 synergizes with erythropoietin to promote erythroid colony formation. IL-9 induces high affinity IgE receptor expression and granzyme A and B in murine T helper clones. The D8402E8 antibody reacts with mouse IL-9.