Forty-three cases of large cell anaplastic thyroid carcinoma were examined with various antisera. Four histological patterns were identified: spindle cell, giant cell, trabecular and squamous. In 38 cases the epithelial origin was demonstrated with various epithelial markers: 11 cases stained positively for thyroglobulin, 19 for T3 and/or T4, 35 for fat globule membrane antigens, 28 for keratin, 29 for lactoferrin, and one for calcitonin. Five cases were negative for all epithelial markers but could not be characterized further since, except for vimentin, they did not have mesenchymal markers. The immunohistochemical proof of the epithelial nature of anaplastic thyroid tumours is given by staining with anti-keratin and anti-fat globule membrane antigen. In addition the detection of lactoferrin seems useful, but not that of thyroglobulin which was present in only 4% of the spindle cell tumours and in 32-56% of the other histological types. Thirty carcinomas were positive for vimentin; the co-expression of vimentin and epithelial markers seems frequent in thyroid anaplastic carcinomas.