Cell suspensions from six anaplastic thyroid tumours were studied for expression of lymphocyte and macrophage surface markers, and results were correlated with electron microscopy, clinical extent of disease, and response to radiotherapy. The clinical presentation of the disease was similar in all six patients. In five cases, many of the cells showed surface immunoglobulin. Electron microscopy was available on three of these and showed appearances in keeping with malignant lymphoma. The single case whose cells did not show surface immunoglobulin had an entirely different ultrastructure and was probably a carcinoma. This case was the only one that did not achieve complete remission with radiotherapy and the patient died from extensive local recurrence. It is concluded that receptor techniques are of value in distinguishing between malignant lymphoma and other anaplastic tumours of the thyroid, particularly when the results are correlated with histology.