The two major transforming proteins of oncogenic human papillomaviruses are encoded by the E6 and E7 oncogenes. Both viral proteins interact specifically with the products of cellular human tumour suppressor genes; E6 with p53 and E7 with Rb. However, the mechanism of action of E6 is still poorly understood in comparison with that of E7. Although extensive in vitro studies have been done with mutant E6 proteins, very little is known about the activities of E6 in vivo. In this study we have analysed the structure-function relationships of HPV-18 E6 in in vitro analyses and we correlate this with in vivo activity. These studies define a number of domains on the E6 molecule which are involved in the ability of E6 to target p53 for degradation in vitro. This analysis demonstrates that domains previously shown to be important in HPV-16 E6 (Crook et al., 1991; Mietz et al., 1992) are also conserved in HPV-18 and also reconciles the differences between these reports. A series of in vivo studies demonstrate that E6 mediated degradation of p53 in vitro is irrelevant both for cell transformation and for the ability of E6 to abolish p53 transcriptional activation. In addition, we show that at least four distinct regions of the E6 protein are involved in the p53 association in vivo.