IL3 is produced mainly by T cells following cell activation by antigens and mitogens, but also by keratinocytes, natural killer cells, mast cells, endothelial cells, and monocytes. The analysis of bacterial-derived recombinant IL3 shows that glycosylation is not required for the activity of IL3. IL-3 sequences are evolutionarily less well conserved. Human and murine IL3 show approximately 29% homology at the protein level while murine and rat IL3 show approximately 54% homology. IL3 receptors are expressed on macrophages, mast cells, eosinophils, megakaryocytes, basophils, bone marrow progenitor cells, and various myeloid leukemia cells. Binding of IL3 to its receptor causes specific phosphorylation of a 150 kDa membrane glycoprotein. Recombinant mouse IL3 is a monomer of 15 kDa.