TrkA, a member of the receptor tyrosine kinase family, binds nerve growth factor (NGF) and subsequently activates intracellular signaling pathways. Previous studies have found variable and weak interaction of the TrkA receptor with neurotrophin 3 (NT-3), another member of the NGF family. TrkA is expressed in two splice forms, differing in the presence of an 18-bp exon in the extracellular domain. The biological responses of each isoform of the TrkA receptor were tested after transfection into the cell line PC12nnr5, a variant of PC12 cells lacking functional TrkA protein. NGF was found to activate each form of the receptor comparably. However, the TrkA isoform containing the variable exon showed significantly higher activation by NT-3, which was detected by stimulation of TrkA autophosphorylation, induction of ZIF268 transcription, and cellular differentiation. Function-perturbing antibodies to the p75 low-affinity NGF receptor potentiated the NT-3 responses of both forms of TrkA in the transfected PC12nnr5 cell lines, suggesting that the low-affinity NGF receptor suppresses the ability of TrkA to respond to NT-3.