Argyrophilic proteins of nucleolar organizers (NO) were measured in 47 benign (24 follicular and 8 atypical adenomas) and malignant (8 papillary and 7 follicular carcinomas) tumors. The content of argyrophilic NO proteins was significantly higher in malignant cells. The proportion of cells with at least 5 silver grains per cell was 16.8 and 22.8% in follicular and papillary carcinoma, respectively, and 3.2 and 12% in follicular and atypical adenoma, respectively. These data can be used as accessory signs in differential diagnosis.