Allogeneic bone marrow transplant followed by donor lymphocyte infusion (DLI) is limited by T cell-mediated graft-versus-host disease (GVHD). A potential solution to alleviate uncontrolled GVHD is to create a controllable suicidal lymphocyte using retroviral transduction of the herpes simplex virus thymidine kinase gene (HSVtk) into the T cell graft. Should GVHD arise, the administration of ganciclovir (GCV) should eliminate the causal T cells. The culture conditions and expansion protocols required to produce suicidal lymphocytes may affect the composition of the T cell product. In this report we describe how T cells from individual donor samples respond to the same culture condition in highly varied ways. Among five donors, two demonstrated predominant expansion of CD4+ cells (with a decrease of CD8+), two donors resulted in predominately CD8+ cells, and one donor developed mainly dual positive CD8+/CD56+ cells. We observed a 20-fold expansion of T cells during the 14 day protocol. The function of the T cells was not affected by the transduction procedures (as tested by 51Cr release assays). In contrast to suicidal lymphocytes prepared using entire T cell populations, T cells pre-selected into CD3+/CD4+ or CD3+/CD8+ subpopulations prior to culture maintained their initial phenotype during the 14 day culture period, with little or no drift. Results from clinical trials using suicidal lymphocytes may be confounded by variance in lymphocyte subset compositions (LSC) and optimal use of suicidal lymphocytes may require separate culture and transduction to control the LSC delivered to the patient.