In contrast to the observed activity of the E7 genes of the genital high-risk human papillomavirus (HPV)16 and HPV18, E7s of the cutaneous high-risk HPV5 and HPV8 show no in vitro transforming activity in established rodent cells. We recently reported that the HPV8 E7 driven by the SV40 enhancer/promoter oncogenically transforms primary rat embryo fibroblast (REF) cells collaboratively with the EJras oncogene (Jpn. J. Cancer Res., 82, 1340-1343, 1991). To study the functional differences between cutaneous HPV5 and HPV8 E7s and genital HPV16 E7, we cloned each of the E7 open reading frames and tested their immortalizing and transforming activities, the binding ability of their products with retinoblastoma protein (RB) and their complementation activity of a RB-nonbinding adenovirus E1A mutant. In contrast to results with HPV16 E7, transfection of HPV5 and HPV8 E7s did not produce any G418-resistant colonies in primary baby rat kidney (BRK) cells. However, they induced morphological transformation of primary BRK cells as well as of primary REF cells when cotransfected with the EJras oncogene. The ras-cooperating activity of HPV8 E7 appears to be extremely low, since, unlike the case of HPV5 and HPV16 E7s, transformed BRK colonies induced by HPV8 E7 plus ras have had a very low survival rate. The in vitro RB binding experiment showed that HPV5 and 8 E7s are able to form complexes with RB protein with reduced affinities of about one fourth and one nineteenth that of HPV16 E7, respectively. Moreover, not only HPV16 E7 but also HPV5 and 8 E7s complemented a nontransforming adenovirus 5 E1A mutant (dl922/947) incapable of binding to RB in inducing E1A-specific transformed foci on primary BRK cells. Since both the activities, the ras-collaborative transformation and complementation of the inert E1A mutant by E7s, all correlate with in vitro RB binding affinity (HPV16 E7 > HPV5 E7 > HPV8 E7), it is likely that RB binding of HPV5 and HPV8 E7s is an integral part of the biological activities of these proteins.