Memory but not naive Id-specific T cells kill Id⁺ B-lymphoma cells by apoptosis. Id⁺ F9 and Id^- F55 cells (104) were tested in a 6- or 18-hour 3H-Td release assay (JAM assay) for susceptibility to killing by unstimulated naive or Th1- or Th2-polarized Id-specific TCR-transgenic SCID splenocytes that had been stimulated in vitro for one (× 1) or 2 (× 2) 10-day cycles. TCR-transgenic lymph node cells served as a source of unstimulated naive CD4⁺ T cells.