The Mdm2 oncoprotein is a well-known inhibitor of the p53 tumor suppressor, but it may also possess p53-independent activities. In search of such p53-independent activities, the yeast two-hybrid screen was employed to identify Mdm2-binding proteins. We report that in vitro and in transfected cells, Mdm2 can associate with Numb, a protein involved in the determination of cell fate. This association causes translocation of overexpressed Numb into the nucleus and leads to a reduction in overall cellular Numb levels. Through its interaction with Numb, Mdm2 may influence processes such as differentiation and survival. This could potentially contribute to the altered properties of tumor cells which overexpress Mdm2.