The genetic regulatory mechanisms that govern tissue specific expression of the T cell receptor (TCR) beta chain have been investigated utilizing an invitro model of cell type specificity. Both transient and permanent transfection systems have been used to assay the transcription of a genomic TCR beta chain gene in T cells, fibroblasts, and a variety of hematopoietic tumor cells. The nature of the vector containing the genomic beta chain gene as well as the relative amount of 5' untranslated sequence contained within the genomic clone influence the degree of tissue specificity observed. TCR beta chain genes containing 5 kb of 5' UT sequence within a vector which contains polyoma enhancer and replication origin sequences display no tissue specificity of expression. By contrast, the same beta chain gene within a pBR vector containing no eucaryotic regulatory sequences shows preferential expression as expected in T cells as compared to fibroblasts. Thus, unlike immunoglobulin genes, tissue specificity of expression as determined by resident regulatory sequences within a genomic TCR beta chain gene are dominantly influenced by external eucaryotic regulatory sequences. In addition, TCR beta chain genes containing 1.5 kb of 5' UT sequence within a pBR vector display no preferential tissue expression. These results imply that a tissue- specific negative regulatory element exists within the deleted 3.5 kb of upstream sequence.