The monoclonal antibody D11 is a valuable aid in the accurate typing of adrenal tumours as, in formalin-fixed, paraffin-embedding material, strong nuclear D11 positivity was observed only in adrenocortical cells in 190 neoplasms (including 100 adrenal tumours). This pattern was demonstrated for all zona glomerulosa cells in 27 normal adrenals and for the neoplastic cells of 15 adrenocortical adenomas derived from that zone, as judged from clinically evident hyperaldosteronism. Normal cells of zona fasciculata and reticularis also showed strong diffuse D11 immunostaining and the same nuclear plus cytoplasmic D11 reactivity was evident in 15 benign and malignant adrenocortical neoplasms derived from these zones, documented by hypercortisolism. Cytoplasmic and/or nuclear D11 staining made topohistogenetic typing possible in 15 non-functioning cortical tumours. D11 immunostaining gave negative results in 50 specimens containing normal, hyperplastic and neoplastic adrenomedullary cells. In addition, absence of D11 reactivity was recorded in 4 adrenal metastases of extra-adrenal carcinomas, 5 paragangliomas, 25 primary renal carcinomas and 59 of 60 primary thyroid carcinomas. D11 immunocytochemistry allows the accurate typing of benign and malignant adrenocortical neoplasms, irrespective of histology and function. With this method, primary adrenocortical tumours can be separated from carcinomas metastatic to the adrenal gland, including secondary tumours of similar phenotype (such as renal carcinomas). By exclusion, D11 negativity provides evidence of the medullary origin of primary adrenal tumours even in the absence of clinical, structural, histochemical and conventional immunohistochemical indicators of phaeochromocytoma.