The catecholic amino acids, dopa and 5-S-cysteinyldopa, and the dopamine metabolite, homovanillic acid, were determined in 8 neuroblastomas. 5-S-Cysteinyldopa and/or dopa were detected in all cases and homovanillic acid was present in 5. Dopa was also found in two tumours in which no definite histological diagnosis could be made. Neuroblastoma cells were cultured from 7 patients. The ageing of human sympathoblasts in culture was accompanied by modifications in the ability to synthetize dopa, which is a precursor of catecholamines as well as of melanins, and the metabolite homovanillic acid. An increase in the levels of 5-S-cysteinyldopa, a metabolite of the melanocytes, has been observed. Concurrent modifications of ultrastructural morphology with the disappearance of granular vesicles and appearance of melanosomes were noticed. This modulation of the original phenotypic expression commonly resulted in cell death, but in one case of metastatic adenopathy of a neuroblastoma we have been able to establish a permanent pigmented cell line.