The SWI/SNF protein complex is required for the enhancement of transcription by many transcriptional activators in yeast. Here it is shown that the purified SWI/SNF complex is composed of 10 subunits and includes the SWI1, SW12/SNF2, SWI3, SNF5, and SNF6 gene products. The complex exhibited DNA-stimulated adenosine triphosphatase (ATPase) activity, but lacked helicase activity. The SWI/SNF complex caused a 10- to 30-fold stimulation in the binding of GAL4 derivatives to nucleosomal DNA in a reaction that required adenosine triphosphate (ATP) hydrolysis but was activation domain-independent. Stimulation of GAL4 binding by the complex was abolished by a mutant SWI2 subunit, and was increased by the presence of a histone-binding protein, nucleoplasmin. A direct ATP-dependent interaction between the SWI/SNF complex and nucleosomal DNA was detected. These observations suggest that a primary role of the SWI/SNF complex is to promote activator binding to nucleosomal DNA.