The prolonged in vivo survival of genetically modified effector cells is crucial to the success of any (gene-modified) adoptive cellular immunotherapy approach. In cancer clinical trials to date, however, the detection of surviving circulating gene-modified T cells has required highly sensitive techniques. In vitro studies of T cell co-stimulation have shown that up-regulation of the anti-apoptosis gene Bcl-X(L) by ligation of CD28 promotes T cell survival, but not proliferation. Here we have investigated the ability to modulate resistance to apoptosis and improve cell survival by transducing human peripheral blood lymphocytes using a retroviral vector that expresses Bcl-X(L). We show that Jurkat cells transduced with Bcl-X(L) retrovirus were partially resistant to Fas (CD95) antibody-induced apoptosis. Subsequent in vitro assays with transduced primary human lymphocytes demonstrates that over-expression of Bcl-X(L) promotes the survival of lymphocytes cultured in the absence of interleukin-2. Activation-induced apoptosis with anti-CD3(epsilon) antibody, OKT3 is also modulated. Furthermore, Bcl-X(L) over-expression in human lymphocytes delays the onset of apoptosis induced by long-term co-culture with tumour cell lines. Despite this improved in vitro survival, in a preliminary experiment to assess safety, no signs of malignancy or autoimmunity were observed in NOD/SCID mice injected with Bcl-X(L) transduced lymphocytes. These results indicate that expression of Bcl-X(L) in lymphocyte therapy either alone or in conjunction with an additional therapeutic gene could enhance persistence of cells in vivo thereby potentially improving the clinical outcome of adoptive cellular therapy.