The introduction of an inducible suicide gene such as the herpes simplex virus thymidine kinase (HSV-TK) might allow exploitation of the antitumor activity of donor T cells after allogeneic hematopoietic cell transplantation (HCT) without graft versus host disease. However, HSV-TK is foreign, and immune responses to gene-modified T cells could lead to their premature elimination. We show that after the infusion of HSV-TK–modified donor T cells to HCT recipients, CD8⁺ and CD4⁺ T-cell responses to HSV-TK are rapidly induced and coincide with the disappearance of transferred cells. Cytokine flow cytometry using an overlapping panel of HSV-TK peptides allowed rapid detection and quantitation of HSV-TK–specific T cells in the blood and identified multiple immunogenic epitopes. Repeated infusion of modified T cells boosted the induced HSV-TK–specific T cells, which persisted as memory cells. These studies demonstrate the need for nonimmunogenic suicide genes and identify a strategy for detection of CD4⁺ and CD8⁺ T-cell responses to transgene products that should be generally applicable to monitoring patients on gene therapy trials. The potency of gene-modified T cells to elicit robust and durable immune responses imply this approach might be used for vaccination to elicit T-cell responses to viral or tumor antigens.