Polyclonal antibodies raised against mouse 2.5S NGF (mNGF) and against synthetic peptides made from hydrophilic portions of mNGF have been used to compare the immunological properties of mNGF, human recombinant brain-derived neurotrophic factor (hrBDNF), and human recombinant neurotrophin-3 (hrNT-3). Affinity-isolated antibodies raised against intact mNGF reacted with all three neurotrophins when tested by ELISA and totally or partially blocked the bioactivities of the proteins in survival assays of embryonic chicken sensory and sympathetic neurons. On Western blots, mNGF antibodies reacted with all three neurotrophins but less well with hrBDNF and hrNT-3 than with mNGF. Antibodies to hydrophilic peptides within NGF (amino acids 23–35, 59–67, 69–79, and 91–100) showed partial reactivity with some but not all of the neurotrophins when tested by ELISA and on Western blots. The peptide antibodies were also selectively effective in reducing the survival- promoting activity of the neurotrophins on sensory neurons. Results show that mNGF, hrBDNF, and hrNT-3 are immunologically related proteins and that mNGF antibodies react also with other members of the neurotrophin family.