A series of 47 primary and seven metastatic thyroid follicular carcinomas, including well, moderately and poorly differentiated, were tested for thyroglobulin (Tg) using immunohistology. In addition, three combined follicular undifferentiated carcinomas, 17 undifferentiated carcinomas and five renal cell carcinomas metastatic to the thyroid were examined. Only two follicular carcinomas did not stain for thyroglobulin. Some inter-tumour differences in Tg staining were found but there was no absolute correlation between this and the degree of tumour differentiation. The two tumours that failed to stain for Tg were poorly differentiated; thyroglobulin positive poorly differentiated tumours demonstrated a clearly weaker staining pattern for Tg. All but one of 15 oxyphilic follicular carcinomas stained positively for Tg but the staining intensity was often weak. Five of six clear cell follicular carcinomas were positive for Tg but the staining reaction was generally faint and there were often large areas devoid of positive cells. Positive staining was demonstrated in the differentiated areas of combined follicular undifferentiated carcinomas. Undifferentiated carcinomas and metastatic renal cell carcinomas gave negative results. Thyroglobulin is a reliable marker for thyroid follicular carcinoma but the patchy staining pattern, particularly in the less well-differentiated tumours, may produce less reliable results in small biopsies.