The "touchdown" polymerase chain reaction (PCR) technique has been applied to analyze expression of the neuron-specific protein, PGP9.5, and tyrosine hydroxylase (TH) genes for detection of minimal residual neuroblastoma cells in bone marrow and peripheral blood. PGP9.5 and TH gene products were not detected in any normal samples (n = 72) examined. However, in patients more than 1 year of age with stage III and IV neuroblastoma PGP9.5 mRNA was detected in six of seven bone marrow samples and in four of eight peripheral blood samples, and TH mRNA in four of seven and three of eight, respectively. The detection sensitivity was up to 10(-6) to 10(-7) micrograms of total cellular RNA for PGP9.5 and 10(-4) micrograms for TH. Among forty bone marrow specimens from nineteen patients with neuroblastoma both PGP9.5 and TH mRNAs were detected in six, and only PGP9.5 mRNA was detected in ten. Since detection of PGP9.5 and TH gene transcripts by the "touchdown" PCR was highly specific and sensitive, it might be most informative at present to carry out both PGP9.5 and TH mRNA assays for minimal residual neuroblastoma cells in blood and bone marrow.