The Rad54 protein plays an important role during homologous recombination in eukaryotes. The protein belongs to the Swi2/Snf2 family of ATP-dependent DNA translocases. We previously showed that yeast and human Rad54 (hRad54) specifically bind to Holliday junctions and promote branch migration. Here we examined the minimal DNA structural requirements for optimal hRad54 ATPase and branch migration activity. Although a 12-bp double-stranded DNA region of branched DNA is sufficient to induce ATPase activity, the minimal substrate that gave rise to optimal stimulation of the ATP hydrolysis rate consisted of two short double-stranded DNA arms, 15 bp each, combined with a 45-nucleotide single-stranded DNA branch. We showed that hRad54 binds preferentially to the open and not to the stacked conformation of branched DNA. Stoichiometric titration of hRad54 revealed formation of two types of hRad54 complexes with branched DNA substrates. The first of them, a dimer, is responsible for the ATPase activity of the protein. However, branch migration activity requires a significantly higher stoichiometry of hRad54, approximately 10 +/- 2 protein monomers/DNA molecule. This pleomorphism of hRad54 in formation of oligomeric complexes with DNA may correspond to multiple functions of the protein in homologous recombination.