Megamodulin, a heat-stable protein from Escherichia coli was isolated and purified near homogeneity as determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis. It had a molecular weight of 71,000 and pl between 3.5 and 4.0. This factor stimulated E. coli RNA polymerase 71-fold in the presence of a synthetic template such as poly (rA).p(dT). When TATAAA sequence was used as template, the RNA polymerase activity was increased 68 times by this factor. The possible mechanism by which this protein factor may regulate the RNA polymerase activity has been described.