Abstract Characteristics of T lymphocytes, including differentiation antigens, capacity to form E rosettes, and proliferative responses to some phytomitogens and to allogeneic stimuli were looked for in peripheral blood lymphocytes from patients with certain immunodeficiency diseases. In vitro induction of some T-cell characteristics on a fraction of bone marrow cells was possible in all normal donors, in patients with Di George syndrome. and in patients with common variable immunodeficiencies, but it was severely altered in all infants with severe combined immunodeficiency diseases (SCID). Although SCID appear to be a very heterogeneous syndrome, some evidence is presented indicating that most cases of SCID may result from a block in early events of T-cell differentiation. After bone marrow transplantation from a compatible donor, T lymphocytes developed from the donor's cells and the sequential stages of maturation confirmed our previously described model based on in vitro experiments: appearance of the HTLA + phenotype, then of the capacity to form E rosettes and later of the responses to allogeneic cells or to phytomitogens. In Di George syndrome treated with fetal thymus transplantation, the recipient's T-cell precursors differentiated and proliferated under the influence of the grafted thymus, following a comparable sequence of events.