The CD3 subunits of the T cell receptor-CD3 complex (TCR-CD3) help to regulate surface TCR-CD3 expression, and participate in signal transduction leading to intrathymic selection and peripheral antigen recognition by T lymphocytes. Humans who lack individual CD3 chains show impairments in the expression and activation-induced downregulation of TCR-CD3, and the defective immune responses that result may be lethal. We have investigated delivery of a normal CD3 chain to treat disorders of this type. Retroviral transduction of CD3gamma into CD3gamma-deficient peripheral blood T lymphocytes from two unrelated patients selectively corrected the observed TCR-CD3 expression and downregulation defects, but unexpectedly seemed to cause adverse effects that can be explained by an autoreactive recognition mechanism. These data support the feasibility of gene therapy for human CD3 deficiencies, but also suggest that gene transfer into postthymic lymphocytes carrying mutations on T cell recognition or activation pathways may disrupt their intrathymic calibration and become harmful to the host.