Although thyroid gland neoplasms are well-recognized entities in dogs, the diagnosis and classification of these tumors often is difficult. In contrast to human thyroid carcinomas, which are predominantly of the papillary or follicular types, a relatively high proportion of the canine tumors contain compact cellular areas and resemble, to some extent, medullary thyroid carcinomas. In order to assess the value of immunohistochemical techniques in the identification and classification of these neoplasms, 21 canine thyroid carcinomas were examined for the presence of thyroglobulin and calcitonin using the peroxidase-antiperoxidase technique. Four major patterns of thyroglobulin immunoreactivity were present in the tumors, including diffuse cytoplasmic positive reaction, apical staining in the cells bordering the neoplastic follicular lumens, intracytoplasmic droplet staining, and staining of intrafollicular colloid. All follicular and mixed compact cellular/follicular tumors contained immunoreactive hormone, while only four of six compact cellular carcinomas were thyroglobulin-positive. The extent of thyroglobulin reactivity was consistently greater in tumors of the follicular and mixed patterns than in carcinomas of the purely compact cellular type. Two of four metastases, each of which retained the mixed pattern of the primary tumors, were thyroglobulin-positive. No medullary thyroid carcinomas were identified, but scattered calcitonin-positive cells in one mixed and in one compact cellular tumor were interpreted as entrapped nonneoplastic C cells. Immunohistochemical localization of thyroglobulin should facilitate the diagnosis of canine tumors of suspected thyroid follicular cell origin, particularly those arising in ectopic sites (i.e., heart base) and those presenting as metastases.