We report the isolation and establishment in continuous culture of a human lymphoid cell line (Peer) from a case of T-leukemia. The Peer cell line lacks some typical cell-surface properties of T cells, namely sheep erythrocyte rosette formation and reactivity with two anti-T-cell sera, but has focal acid phosphatase and does express two other T-cell antigens, one defined by a monoclonal antibody, the other related to a T-cell subset (TH2). The cells are negative for B-cell markers (SmIg or cytoplasmic mu Fcgamma and C3 receptors, mouse erythrocyte rosettes) and EBV (EBNA). In addition, the Peer cell does not possess the typical phenotypic markers of "non-B, non-T" leukemia: cALL and Ia-like antigens, and the cytoplasmic hexosaminidase isoenzyme I, but is positive for terminal deoxynucleotidyl transferase by enzymatic and immunofluorescent criteria. The cell line requires exogenous L-asparagine for adequate growth in culture, a property known to be characteristic of certain T cells but not of B cells. The Peer cell line appears to have a maturation arrest at a developmental stage intermediate between the cortical thymocyte and a mature T-cell subset and to have lost some T-cell differentiation features.