The human Thy-1 gene has been isolated and sequenced and compared to the rat and mouse Thy-1 genes. All three genes are organized in the same way: one exon encoding the majority of the signal peptide, another encoding the transmembrane segment, and a third encoding the remainder of the protein. One major structural difference between the human and rodent Thy-1 glycoproteins is that the former contains two instead of three glycosylation sites. RNA blot analysis of a human T-cell line expressing the T3 complex showed an absence of Thy-1 mRNA, excluding the possibility that Thy-1 represents one of the component chains of T3. The structural gene for human Thy-1 was localized to the long arm of chromosome 11 by nucleic acid hybridization to genomic DNA isolated from somatic cell hybrids.