A T cell receptor (TCR), an αβ heterodimer, recognizes peptide antigens presented by self molecules of the major histocompatibility complex (MHC). A significant number of T cell clonotypes are alloreactive: they can interact with various allelic variants of MHC and the associated peptides. Currently, it is unclear whether the effectiveness of the allogeneic immune response depends on the diversity of the TCR repertoire. Seeking to experimentally narrow the diversity of T cell clonotypes, we used mice with transgenic expression of the β-chain TCR (TCRβ) in this work. We analyzed how the TCRß-transgenic mice on the CBA/Lac (H-2^k) background respond to EL-4 (H-2K^b) lymphoma cells in vivo with the aim to assess the effect of a narrower repertoire on the allogeneic immune response. The study has shown that transgenic mice develop a weak immune response to transplant antigens, and the formed pool of cytotoxic T cells is 1.5–1.7-fold smaller than that in wild-type animals. Consequently, the mice failed to reject the allogeneic tumor, leading to 100% mortality rate. The results of this work are consistent with the data from our earlier studies that employed another TCRß-transgenic model. They confirm that the decreased diversity of the TCR repertoire impairs the response to alloantigens, allowing the tumor to evade the immune response and progress in the allogeneic recipient.