Because activated T cells were previously shown to induce proliferation of human normal B-cell precursors (BCP) via the CD40 pathway, we investigated the effects of T cells on leukemic blasts isolated from patients with B-lineage acute lymphoblastic leukemia (BCP-ALL). An anti- CD3 activated human CD4+ T-cell clone was found to induce significant call proliferation in four of nine BCP-ALL samples analyzed. In one of these cases, the T-cell effect was clearly dependent on interaction between CD40 and its ligand. Accordingly, a more thorough analysis was performed on this particular leukemia (case 461, adult early pre-B-ALL, mBCR+, Philadelphia+, i(9q)+). Thus, autologous CD4+ T cells isolated from the patient were also able to induce CD40-dependent proliferation of the leukemic blasts. Analysis of the phenotype after coculture showed that, among the CD19+ cells, a proportion gradually lost expression of CD10 and CD34, whereas some cells acquired CD23. In addition, and in contrast with normal BCP, activated T cells promoted maturation of a subset of the case 461 leukemic cells into surface IgM+ cells. The leukemic origin of the cycling and the maturing cells was assessed by the presence of i(9q), a chromosomal abnormality associated with this leukemia and evidenced by fluorescence in situ hybridization. Taken together, these results show that leukemic BCP can be activated via CD40 but that not all cases display detectable stimulation in response to T cells despite their expression of CD40. In addition, the present data suggest that CD4+ T cells could potentially play a role in the physiology of BCP-ALL.