Transplanted lymphomas, most of thymic origin, induced in BALB/c mice with 1-ethyl-1-nitrosourea (ENU) and transplanted spontaneously occurring lymphomas of AKR mice were examined for the expression of the T-cell antigens Ly, TL, and Thy 1 by using three serological methods. Most (11 of 13) of the Thy 1+ and/or TL+ tumors, i.e., T-cell tumors, expressed high levels of either Ly 1 or Ly 2 antigen, but not both. Thus most thymic lymphocytic tumors expressed restricted Ly phenotypes comparable to phenotypes previously described for functional peripheral T cells. Because tumor phenotypes were stable over a number of transplant generations, they therefore appeared to be an intrinsic property of the specific tumors. The majority of the BALB/c lymphomas were Ly 1- 2+ and also positive with anti-TL antiserum. This predominant phenotype on the BALB/c tumors may be related to either the mode of tumor induction or to the mouse strain, but since the restricted Ly pattern was observed both in BALB/c and AKR tumors, the phenotypic restriction itself is not a consequence of either of these factors. Tumor induction by ENU per se is not responsible for Ly or TL ,ntigen expression since several non-T-cell BALB/ c tumors, also induced by ENU, did not express either Ly or TL antigens. Data presented here suggest that the target cell for leukemogenesis may be a partially differentiated thymus cell. The restricted expression of Ly antigens on differentiating thymus cells to either the (formula: see text), phenotype may occur before the loss of TL antigen.