SWI5 encodes a zinc-finger protein required for expression of the yeast HO gene. Using Swi5 protein that was purified from a bacterial expression system, we previously isolated a yeast factor that stimulates binding of Swi5 to the HO promoter. N-terminal amino acid sequence analysis identified the Swi5 stimulatory factor as the product of the GRF10 gene, which encodes a yeast homeodomain protein. GRF10, also known as PHO2 and BAS2, is a transcriptional activator of the PHO5 acid phosphatase gene and the HIS4 histidine biosynthesis gene. Grf10 protein purified from a bacterial expression system binds DNA cooperatively with Swi5 in vitro. Analysis of disassociation rates indicates that the Grf10-Swi5-DNA complex has a longer half-life than protein-DNA complexes that contain only Swi5 or Grf10. Finally, we show that HO expression is reduced in yeast strains containing grf10 null mutations and that full expression of a heterologous promoter containing a SWI5-dependent HO upstream activation sequence element requires GRF10.