Rous sarcoma virus-transformed baby hamster kidney fibroblasts (RSVIB4-BHK) adhere to a fibronectin-coated substratum by means of dot-like adhesion sites called podosomes in view of their shape and function as cellular feet (Tarone et al.: Exp Cell Res 159: 14 1, 1985). Podosomes concentrate tyrosine-phosphorylated proteins, including pp60"". and appear in many cells transformed by oncogenes coding for tyrosine kinases. In this paper we used orthovanadate, an inhibitor of phosphotyrosine phosphatases. in order to increase the cellular concentration of phosphotyrosine and to study whether this treatment induced the cytoskeleton remodeling leading to the formation of podosomes. indeed. orthovanadate (10-100 FM) induced in a timeand dose-dependent manner the redistribution of F-actin and the formation of podosomes in BHK cells. Cytoskeleton remodeling occurred along with a marked increase of tyrosine phosphorylated proteins. The vanadate effect on the cytoskeletal phenotype was enhanced by the simultaneous treatment of cells with a phorbol ester. Under the latter conditions almost all BHK cells showed podosomes. The vanadate effect was reversible insofar as podosomes and tyrosine-phosphorylated proteins disappeared. Then, vanadate treatment of normal cells induced the cascade of events leading to the cytoskeletal changes typical of transforniation and suggested that the transformed cytoskeletal phenotype may be primarily induced by the tyrosine phosphorylation of unknown target (s) operated by endogenous kinases.