Serum fucose content in the glycoprotein fraction was determined in various patients with malignant and benign diseases. The results showed that, in contrast to benign diseases, malignant diseases were characterized by an increased fucose content in the glycoprotein fraction. However, no significant difference was noted in the fucose levels in the mucoprotein fraction. The increased fucose level in glycoprotein in malignant diseases was parallel to the increment in total fucose content in serum, which suggests that the increased levels in total fucose in malignant diseases, reported previously, are primarily due to the increase in fucose-containing glycoprotein.