INACTIVATION of tumour-suppressor genes leads to deregulated cell proliferation and is a key factor in human tumorigenesis. Both p53 and retinoblastoma genes are frequently mutated in human cancers(1,2), and the simultaneous inactivation of RB and p53 is frequently observed in a variety of naturally occurring human tumours(3). Furthermore, three distinct DNA tumour virus groups papovaviruses, adenoviruses and human papillomaviruses-transform cells by targeting and inactivating certain functions of both the p53 and retinoblastoma proteins(1,2). The cellular oncoprotein, Mdm2, binds to and downmodulates p53 function(4-6); its human homologue, MDM2, is amplified in certain human tumours, including sarcomas(7-9) and gliomas(10). Overproduction of Mdm2 is both tumorigenic(4) and capable of immortalizing primary rat embryo fibroblasts(11). Here we show that MDM2 interacts physically and functionally with pRB and, as with p53, inhibits pRB growth regulatory function. Therefore, both pRB and p53 can be subjected to negative regulation by the product of a single cellular proto-oncogene.