Several polypeptide factors that are essential for the initiation of protein synthesis bind to eukaryotic mRNAs and facilitate the formation of ribosome initiation complexes. Purified mRNA-binding translation initiation factors were microinjected into quiescent NIH 3T3 cells to study the possible growth-promoting role of these factors in living cells. We report that recombinant eIF-4E and rabbit reticulocyte eIF-4F induce a dose-dependent increase of DNA synthesis and morphologically transform NIH 3T3 cells. These results suggest that polypeptides involved in activating the rate-limiting step of protein synthesis (initiation complex formation) can be mitogenic and oncogenic when overexpressed in a cell by direct injection. Thus, eIF-4E and eIF-4F represent a class of proto-oncogenic proteins that is cytoplasmic, is involved in protein synthesis initiation, and is distinct from the proto-oncogenes that have been identified previously.