NGF is mainly responsible for the survival and the differentiation and the functional activities of sensory and sympathetic neurons in the peripheral nervous system. It also plays an important role in the development and functional activities of cholinergic neurons in the central nervous system. Since NGF is synthesized also in non-neuronal tissues it may have a much wider spectrum of biological activities than thought previously. NGF stimulates chemotactic migration of human polymorphonuclear leukocytes in vitro. NGF stimulates the growth and differentiation of B cells and the growth of T cells and of some tumor cell types. NGF inhibits immunoglobulin production by various human plasma cell. The cytokines IL-1, IL-6, and bFGF are potent inducers of NGF. NGF induces the synthesis of IL-1 in pheochromocytoma cells which in turn acts as a growth factor for glial cells and induces the synthesis of NGF following nerve injuries. In thymic stromal cells NGF induces the synthesis of IL-6. NGF induces the synthesis of the fos oncogene and the myc oncogene and also influences the expression of EGF. One receptor that is responsible for mediating most of the activities of NGF is expressed preferentially in neuronal tissues. The product of the trk gene, NGF expressed in human 293 cells is a non-disulfide bonded homodimeric protein with an apparent molecular mass of 13 kDa. It possesses an intrinsic tyrosine-specific protein kinase in its intracellular domain.