Marrow cells from 16 patients with severe combined immunodeficiency diseases (SCID) were examined for the presence of T cell precursors which could be induced to express surface markers and functions of T lymphocytes after exposure in vitro to thymic extracts or peptides of thymic origin (thymopoietin and thymopoietin32–36 (TP-5)). Marrow cells from 14 patients studied before transplantation revealed three patterns of response. In five patients, inducible T cell precursors were not detected. In six patients, precursors were detected which could be induced to express a human T lymphocyte antigen (HTLA) but acquired little or no ability to rosette with sheep erythrocytes (SRBCs). Induction of HTLA-positive and E rosette-positive lymphocytes was normal in only two patients, both of whom were engrafted with maternal lymphocytes as an apparent result of an intrauterine infusion. Induction of mixed leukocyte reaction (MLR) responses following exposure to thymic hormones was also observed in one of these two patients. In no case could significant responses to mitogens be elicited. Following transplantation, marrow cells from fully engrafted, immunologically reconstituted patients could be induced to bear HTLA, to form E rosettes, and to respond to mitogens and allogeneic cells following exposure to thymic extracts, thymopoietin, or TP-5. Thus, most patients with SCID manifest differentiative abnormalities intrinsic to lymphoid precursors which are corrected following engraftment of functioning allogeneic lymphoid precursors from normal donors.