Neurone-specific enolase, a serum marker of neuroendocrine tumours has been determined in small-cell anaplastic bronchial carcinomas, in typical and atypical carcinoid. The marker presents good sensitivity in small-cell tumours and in atypical carcinoids, while blood levels are within normal limits in cases of typical carcinoid. The low serum levels of neurone-specific enolase found in typical carcinoid suggest a failure of the enzyme to express itself in the circulation owing to the poor or lack of cellular necrosis.