A primary culture of malignant phaeochromocytoma cells obtained from a left adrenal tumor of a 56 year old woman complaining of occasional palpitation and headache was established. The addition of epidermal growth factor (EGF) and insulin to the culture medium induced a profound development of network formation of axon-like processes. At the same time, the secretion of catecholamines from cultured cells was also increased. EGF and insulin were demonstrated to induce the differentiation of malignant phaeochromocytoma cells in primary culture.