Antibodies directed against thyroxine-binding globulin (TBG) have been used to screen a human liver lambda gt11 expression library. A 1.46-kilobase clone was identified which encodes nearly the complete amino acid sequence, beginning at amino acid 17 of the mature protein. To complete the protein sequence, the cDNA clone was used to identify a genomic clone coding for TBG in a human X chromosome library. The overlapping recombinant clones contained an open reading frame coding for 415 amino acids followed by a polyadenylylation signal (AATAAA) located 275 nucleotides from a TAG termination codon. Beginning at residue 21, the deduced amino acid sequence agrees closely with the known NH2-terminal sequence of the mature peptide. The preceding 20 amino acid residues are hydrophobic in character and presumably represent a leader sequence. Four glycosylation sites were identified, corresponding to the number determined for the purified protein. DNA blot hybridization revealed a single-copy gene, which by chromosomal analysis was found to be located on the long arm of the X chromosome. Unexpectedly, the nucleotide sequence of TBG is closely homologous to those encoding the plasma serine antiproteases alpha 1-antichymotrypsin and alpha 1-antitrypsin. However, there is little overall homology between TBG and transthyretin (prealbumin), the other major thyroxine-binding protein of human plasma.