The histological structure of highly and lowly differentiated variants of follicular carcinoma is described. The invasion of the capsule and blood vessels remains the absolute criterion in the evaluation of the degree of malignancy of the struma, but the search for it is quite long, requires examination of large amount of specimens, and is far from being always successful. The test of sex chromatin (SC) which characterises the rate of tumor tissue growth is suggested for differential diagnosis of highly differentiated forms of FC and follicular adenomas. The SC content in follicular adenomas is on the average higher by 10% than in follicular and papillary carcinomas. SC bodies may be counted in any laboratory within a short time.