In dog thyroid glands there are C cell follicles which are lined solely by C cells and which accumulate a colloidlike substance in the luminal cavities. In order to clarify the properties of the colloidlike substance secreted by C cells, the C cell follicles were stained with PAS reaction and immunoperoxidase method using anticalcitonin, anti-C-thyroglobulin, and anti-19S-thyroglobulin antisera, respectively. The colloidlike substance was PAS positive and revealed the strong immunoreaction for C-thyroglobulin but a faint reaction for calcitonin. It was nonreactive with anti-19-thyroglobulin antiserum. These results confirm that C cells synthesize the glycoprotein immunoreactive to anti-C-thyroglobulin antiserum in addition to calcitonin and can store it in the follicular lumens.