Influence of ionic strength on the kinetics of the promoter complex formation between E. coli RNA polymerase and T7 phage DNA was investigated using a membrane filter assay. The enzyme-promoter association rate constant was determined. It varies from 109to 3.107M-1sec-1when the ionic strength is changed from zero to 0.15 M NaCl. Basing on the theoretical analysis of experimental data obtained the model for the promoter site selection assuming the enzyme sliding along the DNA is discussed. © 1980 IRL Press Limited.