Fibronectin extracellular matrix plays a critical role in the microenvironment of cells. Loss of this matrix frequently accompanies oncogenic transformation, allowing changes in cell growth, morphology, and tissue organization. The HT1080 human ®brosarcoma cell line is de®cient in formation of ®bronectin matrix ®brils but assembly can be induced by the glucocorticoid dexamethasone. Here we show that ®bronectin assembly can also be restored by stimulation of a 5 b 1 integrin with activating antibody or with Mn 2+ suggesting that integrin activity is reduced in these cells. While dexamethasone promoted actin stress ®ber formation, actin ®laments remained cortical following Mn 2+ treatment showing that the dexamethasone eect is not due solely to cytoskeletal changes. HT1080 cells have one activated allele of N-ras and PD98059 inhibition of signaling from Ras through ERK increased ®bronectin matrix accumulation. Conversely, the p38 MAP kinase inhibitor SB203580 blocked induction of matrix and increased ERK phosphorylation. Thus, two MAP kinase pathways contribute to the control of integrin-mediated ®bronectin assembly. ERK activity and ®bronectin assembly were linked in three dierent ras-transformed cell lines but not in SV40-or RSV-transformed cells indicating that oncogenic Ras uses a distinct mechanism to down-regulate cell-®bronectin interactions.