The thyroid gland of mammals can be cultivated outside the body. The proliferated elements consist of connective tissue and epithelial cells, the former predominating. The cells survive in cultures for two weeks or longer, which period can be increased by secondary and sometimes by tertiary cultivations. It is to be noted that the method of growing in vitro organs such as the thyroid gland may come to be used with advantage in the study of the substances concerned with the internal secretion of certain glands.