Abstract Expression of α-amylase genes in thyroid tissues was studied by assaying the total amylase activity as well as by using immunohistochemical and Northern blot analysis. The amylase genes expressed were determined by a combination of the polymerase chain reaction (PCR) and blot analysis using synthetic probes specific for the three known amylase isozyme complementary DNAs. The samples consisted of tissues from 18 human thyroid carcinomas (11 well-differentiated carcinomas, 2 poorly differentiated carcinomas, 1 anaplastic carcinoma, and 4 medullary carcinomas) and 9 specimens of nonmalignant thyroid tissue (2 were from nontumorous regions of resected glands and 7 were thyroid tissue from a patient with Graves9 disease). Salivary-type amylase was expressed at a relatively high level in nonmalignant thyroid tissue and well-differentiated carcinoma and could be detected by Northern blot analysis. In poorly differentiated carcinoma, it was detected only by the PCR, while in anaplastic or medullary carcinoma, it was not detected even by the PCR. Thus, the expression of salivary-type amylase was characteristic of well-differentiated follicular cells. These observations suggest that salivary-type amylase expression may be a marker for identifying the histogenesis and stage of differentiation of thyroid cancer. In addition, the AMY2B gene product was detected in all different types of cells examined, although its expression was only detectable by the PCR. Pancreatic type amylase was not detected in any of the samples.