Important insights into cell growth and transcriptional regulation have been realized by correlating adenovirus ElA-induced cell-growth-control mechanisms with ElA’s specific interactions with host-cell proteins (see ref. 4). A key method for directly demonstrating these ElA-host cell protein interactions is by immunoprecipitation. This assay is performed under conditions that efficiently lyse cells, but preserve protein-protein interactions. This method combines the elegant specificities of monoclonal antibodies and the fortuitous properties of nonionic detergents. These highly efficient and specific means of isolating protein complexes from cell lysates, while detailed here for E1Acontaining protein complexes, have general application for the study of other cellular-protein complexes.