Production of proteolytic enzymes is important for the invasive properties of malignant tumours. In this series of 43 cases of various non-neoplastic and neoplastic thyroid lesions, the activity of dipeptidyl peptidase I and dipeptidyl peptidase IV was increased in papillary carcinomas. In addition, cathepsin B and cathepsin L were markedly elevated in 2 of 3 follicular carcinomas, and tryptase, which is a marker of mast cells, was also significantly elevated in follicular carcinomas. Our results indicate that proteolytic proteins are important in thyroid carcinomas, and in addition there seem to be some differences between papillary and follicular tumours. Larger studies are needed to confirm these findings.