The activity of type I and IV collagenase was measured in thyroid tissue obtained from 6 non-diseased thyroids, 4 patients with Graves' diseases, 5 with follicular adenoma, 6 with papillary carcinoma and 4 with follicular carcinomas. The relationship between these enzyme activities and invasion or metastasis of the original tumors was studied. The activity of type I collagenase in papillary carcinomas and follicular carcinomas was higher than in non-diseased thyroids, Graves' disease and follicular adenoma. Carcinoma tissue with invasion beyond the capsule in particular had higher type I collagenase activity. Type IV collagenase activity in carcinoma with lymph node metastasis was higher than in non-diseased thyroids, Graves' disease and follicular adenoma, and especially higher than carcinoma without lymph node metastasis. These findings suggest that increased type I collagenase activity plays an important role in local invasion in thyroid carcinoma, and that increased type IV collagenase activity plays an important role in lymph node metastasis.