HyTK-specific immunity is mediated by HyTK-specific CD8⁺cytotoxic T cells. Postinfusion PBMCs were obtained from UPN 1574, UPN 4103, and UPN 7826 after the HyTK-positive T-cell infusion and stimulated in vitro with γ-irradiated HyTK-expressing T cells twice 1 week apart. Aliquots of these cultures were depleted of CD8⁺ T cells as described in “Patients, materials, and methods” and then evaluated in a chromium release assay for recognition of HyTK-positive target cells (▪) or untransduced LCLs (□). Aliquots of the undepleted cultures (▦) were assayed in a similar fashion for HyTK-positive target cell lysis. Data are shown at an effector-target ratio of 10:1.