Increased lysis of patient CD10-positive leukemic cells by T cells coated with anti-CD3 Fab' antibody cross-linked to anti-CD10 Fab' antibody. An anti-CD3 Fab' x anti-CD10 Fab' bispecific hybrid F(ab')2 antibody (Ab) was generated. This bispecific Ab had a molecular mass of 100 to 110 Kd, and the capacity to react with both CD3+ T cells and CD10+ acute lymphoblastic leukemia (ALL) cells. We studied whether cytotoxic T lymphocytes (CTLs) could lyse patient CD10+ ALL cells after addition of the bispecific Ab. As effector CTLs, interleukin-2 (IL-2)-stimulated peripheral blood mononuclear cells (PBMCs) and CTL clones were used. When IL-2-stimulated PBMCs were assayed for cytotoxicity to 61Cr-labeled CD10+ ALL cells, their activity was shown to be markedly enhanced by the addition of the bispecific Ab. Most of the CTL clones established lacked cytotoxicity for CD10+ ALL cells, but addition of the bispecific Ab induced a significant level of cytotoxicity. CTLs derived from ALL patients also showed significant cytotoxicity for autologous CD10+ ALL cells after addition of the bispecific Ab. However, this Ab did not affect the cytotoxicity of CTLs when CD10- leukemic cells were used as the targets. These findings suggest that the bispecific Ab can be used for immunotherapy in patients with CD10+ ALL. 