A procedure is described for obtaining DNA-dependent RNA polymerase form B from calf thymus in high purity. The procedure includes homogenization in low salt, sedimentation of chromatin at 50 000 x g, adsorption to DEAE-cellulose in a batch procedure, DNA-agarose chromato graphy, and chromatography on DEAE-Sephadex. Analyses of the preparation by band sedimenta tion and gel-electrophoresis in...