The Oct-4 gene encodes a transcription factor that is expressed in embryonic stem (ES) cells and germ cells. Oct-4 is known to function as a transcriptional activator of genes involved in maintaining an undifferentiated totipotent state and possibly in preventing expression of genes activated during differentiation. In addition, it is a putative proto-oncogene and a critical player in the genesis of human testicular germ cell tumors. Although much effort has gone toward characterizing Oct-4, there is still little known about the molecular mechanisms and the proteins that regulate Oct-4 function. To identify cofactors that control Oct-4 function in vivo, we used a recently developed bacterial two-hybrid screening system and isolated a novel ES cell–derived cDNA encoding Ewing's sarcoma protein (EWS). EWS is a proto-oncogene and putative RNA-binding protein involved in human cancers. By using glutathione-S-transferase (GST) pull-down assays, we were able to confirm the interaction between Oct-4 and EWS in vitro, and moreover, coimmunoprecipitation and colocalization studies have shown that these proteins also associate in vivo. We have mapped the EWS-interacting region to the POU domain of Oct-4. In addition, three independent sites on EWS are involved in binding to Oct-4. In this study, we report that Oct-4 and EWS are coexpressed in the pluripotent mouse and human ES cells. Consistent with its ability to bind to and colocalize with Oct-4, ectopic expression of EWS enhances the transactivation ability of Oct-4. Moreover, a chimeric protein generated by fusion of EWS (1-295) to the GAL4 DNA-binding domain significantly increases promoter activity of a reporter containing GAL4 DNA-binding sites, suggesting the presence of a strong activation domain within EWS. Taken together, our results suggest that Oct-4–mediated transactivation is stimulated by EWS.