Apoptosis of normal and leukemic immature B-cells in vitro is suppressed when either cell type is grown in direct contact with a feeder layer of bone marrow-derived stromal cells, including fibroblasts, macrophages, endothelial cells, and adipocytes. In this study, our objective was to identify a stromal cell type which is essential for lymphoblast survival and to characterize the molecules involved in lymphoblast adhesion to these cells. In experiments with B-lineage acute lymphoblastic leukemia (ALL) cells (n = 28) and purified CD19(+) cells from normal bone marrow (n = 6) we found that homogeneous populations of bone marrow fibroblasts could sustain survival of normal and leukemic immature B-cells as efficiently as composite bone marrow stromal layers. Electron microscopic studies showed that leukemic lymphoblasts associate with fibroblasts and with the extracellular matrix (ECM) primarily via their specialized cell surface structures. Immunogold labelingsoliduselectron microscopy analysis revealed that the areas of contact between lymphoblasts and fibroblasts contained beta1 integrins (VLA-4 and VLA-5), fibronectin, vascular cell adhesion molecule (VCAM-1), and a cartilage-link protein, CD44. Double immunogold labeling studies disclosed a direct in situ relationship between fibronectin and VLA-4, VLA-5, and CD44. We hypothesize that these molecular interactions either bring lymphoblasts into close physical proximity with other fibroblast-bound or ECM-bound survival factors or provide survival signals themselves.