We report the first characterization of a mouse T-lymphoma cell line that surprisingly expresses cytoplasmatic (cy) cyCD4. Phenotypically, LBC cells are CD5 , CD8 , CD16 , CD24 , CD25 , CD2^−/dim, CD3^−/dim, TCRβ^−/dim, TCRγδ⁻, CD154⁻, CD40⁻, and CD45R⁻. Coexpress cyTCRβ, cyCD3, cyCD4, and yet lack surface CD4 expression. Transplantation of LBC cells into mice resulted in an aggressive T-lymphoblastic lymphoma that infiltrated lymph nodes, thymus, spleen, liver, ovary, and uterus but not peripheral blood or bone marrow. LBC cells display a modal chromosome number of 39 and a near-diploid karyotype. Based on the characterization data, we demonstrated that the LBC cell line was derived from an early T-cell lymphocyte precursor. We propose that the malignant cell transformation of LBC cells could coincide with the transition stage from late double-negative, DN3 (CD4⁻, CD8⁻CD44^−/low, CD25 ) or DN4 (CD4^−/low, CD8^−/low, CD44⁻, CD25⁻) to double-positive (DP: CD4 CD8 ) stage of T-cell development. LBC cells provide a T-lymphoblastic lymphoma model derived from a malignant early T-lymphocyte that can be potentially useful as a model to study both cellular regulation and differentiation of T-cells. In addition, LBC tumor provides a short latency neoplasm to study cellular regulation and to perform preclinical trials of lymphoma-related disorders.