The interaction between RNA polymerase and the E coli r (ibosomal) RNA promoters of the rrnX and rrnE operon was studied with the filter-binding technique. Quantitative differences were observed between the rrnX and rrnE promoters: stable rrnX promoter complexes are formed faster, and are less sensitive towards heparin and salt than stable rrnE promoter complexes. The effect of ppGpp, the specific inhibitor of rRNA synthesis, on rrn promoter complex formation was studied. In the presence of ppGpp complexes are formed which cannot be trapped in a transcription complex by addition of the start nucleotides, and are therefore considered to be non-productive. A tentative mode for the action of ppGpp is proposed.