Neurotrophic support to peripheral sensory neurons is provided by 2 factors of related sequence, NGF and brain-derived neurotrophic factor (BDNF). NGF is present in peripheral target tissues, while BDNF has only been reported in the CNS. We now report the biological characterization and molecular cloning of a cDNA for BDNF from human platelets. BDNF in human platelets has biological activities very similar to those of BDNF obtained from adult porcine brain in neuron-enriched cultures prepared from peripheral ganglia of chick embryos at 8-12 d of incubation. BDNF from human platelets promoted the survival and neurite outgrowth of placodal and neural crest-derived sensory neurons, but not to parasympathetic or sympathetic neurons. Activity of the factor was additive to that of NGF in dorsal root ganglia (DRG) neuron cultures and is equivalent to porcine brain BDNF in nodose ganglion neuron cultures. On SDS-PAGE, BDNF from human platelets is recovered at an apparent molecular weight equivalent to porcine brain BDNF (13,000 D). A BDNF cDNA fragment was amplified from human platelet RNA by using a coupled reverse transcriptase-polymerase chain reaction. Molecular cloning and DNA sequence analysis of the amplified cDNA fragment revealed complete identity for the deduced amino acid sequences of human and porcine BDNF [amino acid (aa) 10-108 of the mature factor]. Thus, human platelets might provide an important source of BDNF for regenerating peripheral sensory neurons at the site of nerve injury.