DNA-binding proteins from uninfected or polyoma virus-infected mouse cells (3T6) were isolated and compared by DNA-cellulose chromatography and SDS-PAGE. During the early phase of infection, few but significant and reproducible differences were observed. In the 0.4 m NaCl eluate, four major new polypeptides were detected in infected cells, with approximate molecular weights of 160 K, 130 K, 86 K and 81 K, the last two being compatible with previous estimates for polyoma T antigen molecular weight.