Neurotrophins are a family of neuronal survival and differentiation factors (1,2). The mammalian neurotrophin family consists of four members sharing 50–55% amino acid identity: nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and NT-4/5 (2,3). Significant interest surrounds the possible use of neurotrophins for the potential treatment of patients with various neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer’s disease, and Parkinson’s disease, and for lesioned neurons after traumatic injury (bi4). However, successful therapy requires more knowledge of the physiology of the neurotrophins, and precise, reliable determinations of endogenous neurotrophin protein levels are essential for progress in this area.