Tissue cultures of a surgically removed adrenocortical tumour causing Cushing's syndrome, and tissue cultures from the attached, tumour-free adrenal were studied. There were two cell types characteristic of tumour tissue. The cell type occurring most frequently had pronounced hypertrophied agranular endoplasmic reticulum. A fewer number of lipidrich cells containing many electron-dense granules could also be found. The ratio of cells changed during cultivation. In the 17 days tumour culture, a higher percentage of lipid-rich cells could be observed. In spite of continuous ACTH treatment, the initially high hydrocortisone level decreased, gradually. It may be assumed that the lipid-rich cells are of reduced ability as regards hydrocortisone production.