Bispecific CD3xCD19 antibodies and CD28 co-stimulating antibodies were used to activate T cells in bone marrow aspirates (n ؍ 8) of children with B cell-derived acute lymphoblastic leukemia. Bone marrow specimens were incubated for 10 days with CD3xCD19 bispecific and CD28 antibodies. Changes in the numbers of T lymphocytes and tumoral B cells as well as surface expression of T cell-activation markers were determined by flow cytometry, and cytokines (human IFN-␥, IL-2, IL-4 and IL-12) were measured in the cell culture supernatant. In 7 of 8 bone marrow samples, an increase in the number of CD4-and CD8-positive T lymphocytes was found, which correlated with an up-regulation of T cellactivation markers. Additionally, we demonstrated a decrease of tumoral B cells in 3 samples and enhanced cytotoxic T-cell activity against autologous malignant B cells. ELISpot analyses in an autologous Epstein-Barr virus model showed that bispecific antibodies (CD3xCD19؉CD28) were more potent at generating T-cell responses against autologous and allogeneic tumoral targets than a combination of monospecific antibodies (CD3؉CD28). Thus, T-cell targeting by CD3xCD19 bispecific and CD28 antibodies may be used to eliminate leukemic B cells ex vivo and reconstitute immunological control of residual malignant disease by the induction of anti-tumoral T-cell responses. Int.