The specificity of the T-cell receptor (TCR) and its interaction with coreceptors play a crucial role in T-cell passing through developmental checkpoints and, eventually, determine the efficiency of adaptive immunity. The genes for the α and β chains of TCR were cloned from T-cell hybridoma 1D1, which was obtained by fusion of BWZ.36CD8α cells with CD8⁺ memory cells specific for the H-2K^b MHC class I molecule. Retroviral transduction of the 1D1 TCR genes and the CD4 and CD8 coreceptor genes was used to obtain 4G4 thymoma variants that exposed the CD3/TCR complex together with CD4, CD8, or both of the coreceptors on their surface. Although the main function of CD4 is to stabilize the interaction of TCR with MHC class II molecules, CD4 was found to mediate the activation of transfected cells via TCR specific for the H-2K^b MHC class I molecule. Moreover, CD4 proved to dominate over CD8, since the response of CD4⁺CD8⁺ transfectants was suppressed by antibodies against CD4 and the A^b MHC class II molecule but not to CD8. The response of CD4⁺ transfectants was not due to a cross-reaction of 1D1 TCR with MHC class II molecules, because the transfectants did not respond to splenocytes of H-2^b knockout mice, which were defective in the assembly of the MHC class I molecule/β2 microglobulin/peptide complex and did not expose the complex on cell surface. The domination was not due to sequestration of p56^lck kinase, since CD4 devoid of the kinase-binding site was functional in 4G4 thymoma cells. The results were used to explain some features of intrathymic cell selection and assumed to provide an experimental basis for developing new methods of anticancer gene therapy.