The bovine papillomavirus type 1 (BPV) E5 protein is a 44 amino acid, membrane-associated protein that induces growth transformation of cultured rodent and bovine fibroblasts. In transformed fibroblasts, the BPV E5 protein activates the endogenous platelet-derived growth factor (PDGF) beta receptor, and the introduction of the PDGF beta receptor gene into heterologous cell types normally lacking PDGF beta receptor expression permits transformation by the E5 protein. However, neither the endogenous PDGF beta receptor nor its signaling pathway has been shown to be required for efficient growth transformation of fibroblasts by the BPV E5 gene. Here we have tested whether the endogenous PDGF beta receptor serves as a target for the BPV E5 protein in mouse C127 fibroblasts. We isolated variant C127 cell lines that displayed reduced DNA synthesis in response to PDGF but a normal response to other mitogens, suggesting that they harbor specific defects in the PDGF signaling pathway. The variant lines also exhibited a specific reduction in the level of DNA synthesis induced by the acute expression of the BPV E5 gene, and a variant cell line containing a reduced level of PDGF beta receptor also displayed reduced stable growth transformation by the BPV E5 and v-sis oncogenes. These results provide genetic support for the model that the PDGF beta receptor signaling pathway is required for efficient growth transformation of C127 cells by the BPV E5 gene.