We have analyzed the genomic organization of the murine CD3 eta gene. As suggested from the sequence of CD3 eta cDNA, exons I-VII are identical to those of CD3 zeta and exon VIII is a unique exon for CD3 eta. Exon VIII of the CD3 eta gene is located 4 kb downstream of exon VIII of the CD3 zeta gene. Together with the existence of a splicing acceptor site just before exon VIII of the CD3 eta gene, the CD3 eta chain is proved to be produced by an alternatively spliced transcript of the common gene of CD3 zeta and eta. Since T cell receptor complex contains two forms of zeta dimers, zeta-zeta and zeta-eta, on a single T cell, which seem to have different functions, the results indicate the importance of analyzing the regulation of the alternative splicing to understand T cell function.