Abstract The ability of Escherichia coli RNA polymerase to transcribe chromatin from mouse and frog (Xenopus laevis) liver was examined. In mouse liver chromatin, E. coli polymerase did not transcribe the mouse satellite sequences, although it did transcribe them in deproteinized DNA. In confirmation of previous reports, unlabeled mouse liver nuclear RNA competes more efficiently for the hybridization of chromatin transcripts to bulk DNA than it does with transcripts of deproteinized DNA. However, when chromatin and DNA transcripts were hybridized to mouse DNA from which the satellite sequences had been removed, they hybridized and competed identically. It is concluded that this type of hybridization-competition experiment is a very insensitive way to assay the fidelity of chromatin transcription. In chromatin from the frog, both the genes for rRNA and the genes for 5 S RNA are transcribed but in an aberrant manner. Transcription occurred on both strands and in the gene as well as spacer regions. In the F1 hybrid between X. laevis andX. mulleri, transcription of theX. mulleri rDNA † is repressed by over 95% in vivo (Honjo & Reeder, 1973). In chromatin from such hybrid frogs, E. coli RNA polymerase transcribed at least 25% X. mulleri rRNA, indicating that the bacterial polymerase could not recognize the repression mechanism. These studies suggest that transcription of chromatin by E. coli RNA polymerase is much less accurate than previously claimed.