DBA/2 mice which are neonatally infected with Rauscher helper virus (R-MuLV) develop predominantly lymphatic leukemias. From one of these lymphatic leukemias we established a permanent cell line which we named RLD (Rauscher Lymphoid DBA/2). Phenotyping of this cell line with a panel of monoclonal antibodies directed to cell-surface determinants shows that RLD cells have T-cell characteristics: they bind monoclonal antibodies directed to the antigens Thy-1, T-200 and Lyt-1; they do not react with anti-Lyt-2 antibodies, nor do they react with antibodies directed to determinants on B cells or myelomonocytic cells. RLD cells show a high activity of the nuclear enzyme terminal deoxyribonucleotidyl transferase (TdT). RLD cells are able to differentiate after in vitro stimulation with 1% DMSO or with 30 nM tetradecanoylphorbol-1.3-acetate (TPA). This differentiation process is reflected by (1) changes in the 2D gel electrophoresis pattern of metabolically labelled proteins, (2) a decrease in TdT activity and (3) changes in the expression of cell-surface markers. Flow cytometric analysis of stimulated RLD cells shows a strong increase in the Lyt-1 expression. Together these data indicate that RLD cells are immature T lymphocytes which upon appropriate stimulation differentiate along the line of T helper cells.