A large multisubunit complex containing RNA polymerase II, general transcription factors and SRB regulatory proteins initiates transcription of class II genes in yeast cells(1-4). The SRB proteins are a hallmark of this RNA polymerase II holoenzyme as they are found only in this complex, where they contribute to the response to regulators(4-8). We have now isolated a human homologue of the yeast SRB7 gene and used antibodies against human SRB7 protein to purify and characterize a mammalian RNA polymerase II holoenzyme containing the general transcription factors TFIIE and TFIIH. This holoenzyme is more responsive to transcriptional activators than core RNA polymerase II when assayed in the presence of coactivators.