mRNAs were isolated from 2 patients suffering from a familial form of a rare variant of medullary carcinoma of the thyroid (MTC), called mixed follicular and medullary carcinoma. The presence of calcitonin (CT) and thyroglobulin (Tg) mRNAs was checked by northern and in situ hybridization and compared with immunohistochemical results. In each case, mRNAs hybridizing to probes specific for CT and Tg were detected. Both proteins were quantified by radioimmunoassay determination in tissue extracts. Patient 1 had 20 ng Tg and 68 ng CT per micrograms total protein, and patient 2 had 0.4 ng Tg and 1.7 ng CT per micrograms total protein. Northern analysis showed that mixed carcinoma expressed several species of both CT mRNAs and Tg mRNAs. The main Tg transcripts present in neoplastic cells (8.5 and 4.8 kb for patient 1 and patient 2) were identical to or smaller than those of normal thyroid tissue (8.5 kb). The tumor CT mRNA (1 kb) was identical to that of normal tissue. In situ hybridization confirmed the presence of CT and Tg mRNA in the great majority of tumor cells. Furthermore, the presence of small amounts of organified iodine was evidenced by analytical ion microscopy in 35% of these cells. This raises an important question regarding the histogenesis of this tumor.