Many cellular and some virally coded proteins regulating key events in higher cells have been found to be multifunctional. One example of such a protein is the polyomavirus large T antigen, which is involved not only in viral DNA replication and gene expression but also in the induction of the S phase in host cells, in the immortalization of various cell types and in the transactivation of some cellular genes. We recently constructed cell lines in which T antigen was synthesized under the control of a hormone-inducible promoter. This allowed us to induce different concentrations of the protein in the cell and to investigate the levels of large T antigen required for replication, S-phase induction and transactivation of a growth-regulated promoter. We found that significantly higher concentrations of large T antigen are required for the replication function of the protein than for either S phase induction or transactivation. These observations for the first time provide clear evidence for a potential regulation of different functions of a pleiotropic protein by the amount of that protein produced in the cell.