Spi-1/PU.1 and Spi-B encode hematopoietic-specific transcription factors that are the most distantly related members of the Ets family. The Ets proteins share a conserved 85 amino acids DNA binding domain, the Ets domain and recognize various DNA target sites around a common core 5'-GGAA/T-3'. The DNA binding specificities of Spi-1 and Spi-B were investigated by using the method of polymerase chain reaction (PCR)-mediated random site selection. The deduced Spi-1 and Spi-B consensus binding sites are very similar suggesting that the functional activities of Spi-1 and Spi-B cannot be distinguished on the basis of their DNA binding specificities. We identified a putative Spi-1/Spi-B binding site in the promoter region of the c-fes/c-fps protooncogene which encodes a tyrosine kinase expressed predominantly in myeloid cells. In vitro translated Spi-1 and Spi-B proteins were capable to bind this site similarly and to activate the c-fes promoter in HeLa transfected cells. We showed that Spi-1 binds the Spi-1/Spi-B binding site of c-fes in HL-60 cells suggesting that Spi-1 may be involved in the regulation of c-fes transcription in myeloid cells. Intriguingly, we detected only Spi-1 binding to this site in the Raji cell line which express both Spi-1 and Spi-B proteins. This suggests that Spi-1 and Spi-B exhibit different DNA binding activities in vivo although they share similar DNA binding specificities in vitro.