The role of cAMP receptor protein (CRP) in transcription activation at the Escherichia coli lac promoter was investigated focusing on the steps after the formation of open complex. Although CRP binding to the lac DNA is stabilized in the ternary open complex, a high concentration of heparin dissociates CRP from the open complex without affecting the interaction between RNA polymerase and promoter, resulting in a binary complex. The release of CRP is directly shown by Western blotting and DNase I footprinting. The binary complex exhibits a slightly increased gel mobility compared to the ternary complex. The binary complex retains the characteristics of the open complex in footprinting pattern which is essentially identical with that of the open complex of the lac UV5 promoter. The binary complex is competent for transcription. These results indicate that CRP is not necessary for the maintenance of active open complex. In addition, the removal of CRP does not increase the production of abortive RNAs. We conclude that the contact between CRP and RNA polymerase is not essential for transcription activation after the formation of the open complex at the lac promoter. In other words, the role of CRP in the lac promoter is restricted to the steps up to the formation of open complex.