DNA-dependent RNA polymerase has been purified from E. coli B grown to three-quarters log phase in minimal medium. The method of purification includes chromatography on Agarose 1.5 M, and QAE-Sephadex. The second of these steps resolves the RNA polymerase activity into two fractions. One of these has the composition (ββ′σα2). The other contains the "core" subunits β, β′, and α, and another protein whose molecular weight is 68 000 in sodium dodecyl sulfate. This latter protein can be separated from the core subunits, and is shown to have ATPase activity. The possible relationship of this protein to ones of similar molecular weight, found in RNA polymerase preparations of other authors, is discussed.