FRTL-5 cells were cultured in media containing standard growth factors with or without TSH, plus labeled precursors of N-linked oligosaccharide chains. The thyroglobulin secreted in the medium was purified and fragmented with CNBr. Three peptides were identified by NH2-terminal sequencing, that were labeled mainly with D-[2-3H]mannose, independent of TSH. One of them, corresponding to the NH2-terminus of thyroglobulin, incorporated both more D-[2-3H]mannose and more D-[1-3H]galactose upon TSH addition. These data likely reflect a TSH-induced increment of N-linked glycosylation at the NH2-terminus of thyroglobulin, mostly with the maturation of high-mannose to complex chains.