Nerve growth factor (NGF) is a neurotropic polypeptide which has broad biological activity other than support of growth and survival of sympathetic, sensory and central neurons. NGF promotes rat mast cell hyperplasia in vivo and human granulopoiesis in vitro, selectively augmenting basophil/mast cell differentiation in the presence of T cells or conditioned medium derived from a human T cell line (Mo-CM), a source of granulocyte-macrophage colony-stimulating factor (GM-CSF). NGF also synergizes with GM-CSF to promote human basophil/mast cell differentiation in both methylcellulose and suspension cultures of myeloid progenitors. In the current studies, we examined the interactions of NGF and several cytokines considered to be involved in human basophil/mast cell and eosinophil growth and differentiation, including interleukin (IL)-3, IL-4, IL-5, GM-CSF and granulocyte colony-stimulating factor (G-CSF). NGF synergistically enhanced IL-5 induced dose-dependent increases in histamine content and basophilic cell differentiation of myeloid leukemic HL-60 cells, but was only additive to similar effects of IL-3. In contrast, IL-4 and G-CSF did not promote basophilic differentiation of HL-60 cells in the presence or absence of NGF. Various combinations of GM-CSF, G-CSF, IL-3, IL-4 and IL-5 could not reproduce the synergy observed between NGF and either IL-5 or GM-CSF. NGF appears to represent a class of lineage-specific co-factors, in this case being involved in GM-CSF- or IL-5-induced basophilic lineage differentiation, thus contributing to tissue inflammation or repair.