Interactive signaling between cancer cells and stroma plays an important role in determining tumor development. We recently found tumor cell-derived factors to induce fibroblast clustering, and that the high amounts of hepatocyte growth factor/scatter factor (HGF/SF) produced by these cell–cell contact-activated fibroblasts enhanced the invasiveness of c-Met-expressing cancer cells. We now observed that leukemia cells lacking c-Met respond to this novel type of fibroblast activation, nemosis, with growth arrest and differentiation to a dendritic cell-like phenotype. This effect was counteracted by introduction of c-Met expression into these cells. Moreover, those leukemia cell lines harboring properly processed c-Met showed no effect in response to nemosis. We propose this effect to be mediated by nemosis-derived cytokine signals, and present the potential candidates induced in the nemotic fibroblasts: interleukins-1, -6, -8, -11, leukemia inhibitory factor and granulocyte-macrophage-colony-stimulating factor. Our results emphasize the role of activated stromal fibroblasts in controlling progression of certain hematologic malignancies in a c-Met expression-dependent manner. © 2007 Wiley-Liss, Inc.