Approximately 20% of B-lineage acute lymphoblastic leukemias are not cured by traditional chemotherapy. The possibility was examined that residual leukemic cells that potentially contribute to relapse are harbored in association with fibroblastic stromal cells in the bone marrow. Modulation of cytarabine (Ara-C) and etoposide (VP-16) efficacy by bone marrow stromal cells in vitro was investigated. Stromal cell coculture was shown to sustain the proliferation of B-lineage leukemic cells and to reduce leukemic cell apoptosis when exposed to Ara-C or VP-16. Direct contact with stromal cells was essential for the protection of leukemic cells during chemotherapy, whereas soluble factors had negligible effect. Specifically, signaling mediated through interaction with the stromal cell adhesion molecule VCAM-1 was required to maintain the maximum viability of leukemic cells during Ara-C and VP-16 exposure. In contrast, the interaction of leukemic cells with fibronectin did not confer significant resistance to either chemotherapeutic agent. These observations suggest a role for the bone marrow microenvironment in modulating the response of B-lineage leukemic cells to Ara-C or VP-16, and they indicate specific molecular interactions that may be important in determining the sensitivity of leukemic cells to treatment.