Bcl-2 and Bax proteins, which are involved in repressing and promoting programmed cell death, respectively, have been investigated immunohistochemically and by Western blot analysis in a series of thyroid tumours. Three immunostaining patterns were identified. Benign lesions and well-differentiated thyroid carcinomas displayed a profile similar to that of normal follicular epithelium, in which Bcl-2 immunostaining was predominant. Thyroid carcinomas associated with an aggressive behaviour, such as the tall-cell variant of papillary carcinoma and the poorly differentiated carcinomas, co-expressed both proteins. Finally, anaplastic carcinomas expressed only the Bax protein. Western blot analyses revealed that the anti-Bcl-2 antibody recognized two bands, of molecular weights 21 kDa and 25 kDa. This was only seen in the tall-cell papillary carcinomas and in the anaplastic carcinomas.