The human papillomavirus-16 (HPV16) E5 gene is able to induce stable growth transformation and transient mitogenic stimulation in a variety of cultured cell systems. To characterize the biochemical properties of the hydrophobic HPV16 E5 transforming protein, we have constructed vectors expressing the wild-type HPV16 E5 gene and have generated antipeptide antisera. The 10-kDa E5 protein was readily detectable in transfected COS monkey cells by using these antisera either for immunoprecipitation of metabolically labeled cells or for immunoblotting. Coimmunoprecipitation analysis of cells coexpressing the viral protein and various growth factor receptors demonstrated stable complex formation between the E5 protein and the epidermal growth factor receptor, platelet-derived growth factor beta receptor, colony stimulating factor-1 receptor, and p185neu. The E5 protein also formed a stable complex with the vesicular stomatitis virus glycoprotein. These experiments indicated that the HPV16 E5 protein was able to participate in complex formation with a variety of transmembrane proteins, a property which may contribute to the biological activities of the viral protein. In addition, the expression vectors and antibodies described here will be useful reagents in examining various aspects of HPV16 E5 expression and function.