Functionally rearranged T cell receptor alpha and beta-chain genes from a fluorescein-specific cytotoxic T cell clone have been introduced, together with the selectable marker gene neo, into mouse fibroblasts (L cells) by electroporation. Transformed cells were selected for neo gene expression by growth in medium containing the antibiotic G418. Southern blot analysis of DNA from transformed L cell clones revealed that the endogeneous T cell receptor alpha and beta-chain genes were in germ-line configuration and that in 4 of the 6 clones examined the exogenously introduced rearranged alpha and beta chain genes were present. The introduced beta-chain gene is transcriptionally active in two L cell clones examined whereas no transcription of the alpha-chain gene could be detected in the same transformants.