The presence of tumour cells in peripheral blood of neuroblastoma patients is of considerable clinical importance. Nucleic acid amplification offers an opportunity to detect very small numbers of such cells, but in neuroblastoma a frequent specific abnormality in the tumour DNA suitable for this purpose has yet Co be identified. To facilitate the detection of such cells we have developed RT-PCR using tyrosine hydroxylase (TH) as a tissue-specific target gene. TH mRNA was detected in 3 neuroblastoma cell lines and in all neuroblastoma tumours examined, but was undetectable in peripheral blood from children without neuroblastoma. The method was highly sensitive, detecting 1-10 neuroblastoma cells per 107 blood cells. Thirty blood samples from 24 patients were analysed and results were compared with known disease status. At diagnosis 4/7 patient blood specimens were positive; the four positive samples were from stage-4 patients. In blood samples from these patients 6-8 weeks after the initiation of treatment, TH mRNA was undetectable. Of 7 samples taken at the time of clinical relapse, 5 were positive; 4 of these were from patients with evidence of disseminating disease. Of 16 blood samples from disease-free patients, 14 were negative and 2 were positive. One positive patient in this group subsequently had a clinical relapse. These results show that this technique is of value for detecting neuroblastoma cells in peripheral blood. The significance of these cells at diagnosis, during treatment or on follow-up requires further evaluation.