The nature of the acid-soluble mannosamine intermediates was investigated in bovine thyroid gland slices after incubation with a tracer amount of [14C]mannosamine plus 10 mM carrier mannosamine. 2. These radioactive intermediates were separated by ion exchange, columns, characterized by paper chromatography and were identified as N-acetylmannosamine-6-P. The radioactivity associated with these acid-soluble compounds constituted about 17 percent of the added [14C]mannosamine. 3. [14C]Mannosamine was incorporated as sialic acid only into thyroglobulin.