Lymphokine produced by antigen- or mitogen-activated T lymphocytes, which stimulates proliferation of hematopoietic as well as lymphoid stem cells; a colony-stimulating factor for all bone marrow progenitor cells. IL-3 supports the growth and differentiation of early hematopoietic and lymphoid stem cells as well as that of more mature hematopoietic cells, including granulocytes, macrophages, and mast cells