Interleukin-3 (IL-3), interleukin-5 (IL-5), and granulocyte macrophage-colony stimulating factor (GM-CSF) are cytokines that bind to receptor complexes comprised of unique alpha-receptor subunits specific for each ligand and a commonly shared beta-receptor subunit. Previous studies have shown that IL-3 and GM-CSF induce mitosis in microglia and macrophage cells, indicating the functional presence of their cognate receptors. In this study, it is shown that the third member of this cytokine group, IL-5, also serves as a microglia mitogen. Proliferative effects were seen in culture on both murine microglia and a murine macrophage cell line, RAW 264.7. Since IL-5 is known to be secreted by both microglia and astrocytes in response to inflammatory stimuli, these results indicate that IL-5 may be involved in the cytokine-immune cascades leading to microglia proliferation in areas affected by disease and tissue damage.