Paraffin-embedded specimens of 200 primary thyroid carcinomas were examined immunohistologically for the expression of intermediate-filament (IF) protein of the cytokeratin, vimentin and neurofilament type. In 36 cases, snap-frozen tissue was available, and double label immunofluorescence microscopy was performed in 23 of them. Cytokeratin reactivity was found in all cells of all follicular, papillary and medullary carcinoma cases examined. Using a monoclonal vimentin antibody, positive staining was found in many, though not all cells of the papillary tumours and in approximately 50% of the follicular and the medullary carcinomas. Among anaplastic carcinomas, some tumours were positive for cytokeratins, with or without coexpression of vimentin. Neurofilaments could only be demonstrated in approximately 13% of medullary tumours which in general also exhibited vimentin positivity. The differences of IF expression in follicle and C-cell thyroid carcinomas and the broad variation of cytokeratin and vimentin immunoreactivity among anaplastic tumours of this organ is discussed in relation to the possible intrinsic heterogeneity of these tumours and the diagnostic value of these markers.