To carry out its transformation function, the middle tumor antigen (MT) of murine polyomavirus associates with a number of cellular proteins involved in regulation of cell proliferation, including pp60(c-Src), phosphatidylinositol 3-kinase, protein phosphatase 2A, Src homologous and collagen protein and growth factor receptor-binding protein 2. Here, two additional MT-associated proteins were identified as members of the 14-3-3 family of proteins. Yeast homologs of 14-3-3 proteins have recently been shown to play a role in the timing of mitosis. Thus, regulation of 14-3-3 protein function by MT may contribute to the development of neoplasia.