Forty-three fresh tumor specimens of human neuroblastoma belonging to different clinical stages were analyzed for the expression of 2 proto-oncogenes: trk, which encodes a tyrosine-kinase receptor for nerve growth factor (NGF) and ret, another receptor-type tyrosine kinase whose ligand is unknown. The mRNA expression of the trk gene was detected in 67.4% of cases, with increased frequency in I, II and IVs Evans' stages and in patients with favorable prognosis according to the Shimada classification. Moreover, trk expression inversely correlated with Nmyc-gene amplification. ret mRNA was found in 36.8% of cases and equally distributed in the different stages. In addition, ngfR (low-affinity NGF receptor)-gene expression was present in 9 out of 25 cases. The simultaneous presence of mRNA related to both forms of the NGF receptor, while not proving the presence of a functional receptor, indicates the existence of a sub-set of neuroblastoma cells potentially responsive to NGF.