The Ly-6 alloantigens have been shown to play a critical role in T lymphocyte activation. To isolate a Ly-6 cDNA, synthetic oligonucleotides, based on the partial amino acid sequence of purified Ly-6E.1 protein, were used to probe a cDNA library. The synthetic oligonucleotides or the isolated cDNA detected a 1.1-kb RNA species. Sequence analysis of the cDNA clone revealed that the Ly-6E.1 protein consists of a 26-amino acid leader followed by a 108-residue, cysteine-rich, core protein with no N-linked glycosylation sites. Southern blot analysis of genomic DNAs revealed multiple bands indicating a family of related genes. Using recombinant inbred and Ly-6 congenic strains of mice, restriction fragment length polymorphisms were demonstrable, and correlated with the Ly-6 allotype of the DNA donors. This probe will enable further molecular genetic analysis of the role of Ly-6-linked proteins in the process of T lymphocyte activation. Isolation of Ly-6 genomic clones may promote a further understanding of the complex tissue-specific expression patterns characteristic of Ly-6-linked genes.