Oncogenically transformed cells show reduced assembly of fibronectin-rich extracellular matrixes and diminished ability to adhere to fibronectin. The molecular bases of these phenotypic alterations are not fully understood. We report here alterations in the spectrum of integrins, including two fibronectin receptors, on oncogenic transformation of rodent cells. Transformation of rat1, NRK, and Nil8 cells by Rous sarcoma virus or by murine sarcoma viruses encoding ras oncogenes leads to reductions in the level of integrin α₅β₁, which is a well-defined fibronectin receptor, and of two other integrin receptors. In contrast, another receptor, α₃β₁, which is a polyspecific receptor for fibronectin, laminin, and collagen, is retained by transformed cells. These results provide explanations for earlier results concerning the interactions of extracellular matrix proteins with the surfaces of tumor cells and offer leads to further understanding of the altered adhesive and migratory behavior of malignant cells.