The biosynthesis of thyroglobulin has been investigated in five welldemarcated thyroid medullary carcinomas. The biochemical methods used allowed the study to be performed on small amounts of tissue, about 2–4 mg. The biochemical results were compared with the morphological structure. A normal biosynthesis was obtained in three of the five investigated tumours. No indication of a biosynthesis was obtained in the remaining two tumours. The morphological investigation revealed that normal thyroid follicles were found in the tumour tissue close to the pieces with a normal biosynthesis. Normal thyroid follicles could not be found in the tumour tissue with no sign of biosynthesis. Metastases from two medullary carcinomas were also investigated. No indication for a biosynthesis of thyroxine or thyroglobulin was found. Normal thyroid follicles could not be demonstrated. It is concluded that the biosynthesis of thyroglobulin in well-demarcated medullary carcinomas occurs in normal thyroid tissue present within the tumour. It is also concluded that all studies on the function of thyroid carcinomas must involve studies on very small pieces of fresh tissue in order to avoid heterogeneity and must involve a morphological examination of the surrounding tissue.