A cellular phosphoprotein with an apparent molecular mass of 90 kd (p90) that forms a complex with both mutant and wild-type~ 53 protein has been characterized, purified, and identified. The protein was identified as a product of the murine double minute 2 gene (mdm-2). The mdm-2 gene enhances the tumorigenic potential of cells when It is overexpressed and encodes a putative transcription factor. To determine if mdm-2 could modulate~ 53 transactivation, a p53-responsive element from the muscle creatine kinase gene was employed. A wild-type p53-expressing plasmid enhanced the expression of the pl&responsive element when cotransfected into cells that contain no endogenous~ 53. When a cosmid expressing mdm-2 was transfected with this p53-expressing plasmid, the transactivation of the p53-responsive element was inhibited. Thus, a product of the mdm-2 oncogene forms a tight complex with the~ 53 protein, and the mdm-2 oncogene can inhibit p53-mediated transactivation.