Nearly all cases of Ewing Family Tumors (EFT) harbor chimeric EWS/ETS transcription factors which are thought to aberrantly regulate transcriptional targets of phenotypic consequence. We have recently demonstrated that EWS/ETS proteins up-regulate platelet derived growth factor-C (PDGF-C), a novel transforming growth factor. To determine if PDGF-C signaling contributes to the malignant phenotype of EFT cell lines, we attempted to disrupt this presumed autocrine loop. AG1296, a PDGF receptor selective tyrosine kinase inhibitor, markedly inhibits anchorage-independent growth in an EFT cell line. To effect specific disruption, we have developed a dominant negative form of PDGF-C which is appropriately secreted and processed. This mutant has greatly reduced activity as a PDGF receptor agonist. When co-expressed with PDGF-C in a fibroblast transformation model, this dominant negative dramatically inhibits anchorage-independent growth. When this mutant is expressed in EFT cell lines, there is a similar reduction in anchorage-independent growth. This demonstrates that specific inhibition of PDGF-C signaling in EFT cell lines partially reverts their phenotype. These data support a significant role of PDGF-C in the biology of EFT. They also suggest that PDGF-C driven signaling may be a possible therapeutic target of more clinically relevant tyrosine kinase inhibitors.