Functionally equivalent subunits of RNA polymerase from Micrococcus luteus and Escherichia coli differ from each other in many molecular and antigenic properties. In spite of these differences, subunit alpha from E. coli and subunit beta from M. luteus form a complex alpha2beta, when incubated together. This complex binds rifampicin tightly, which the isolated subunits do not. The hybrid complex is very similar in its properties to the complex alpha2beta formed only from E. coli or M. luteus subunits. Since the sub-assembly alpha2beta from E. coli is reported to be an obligatory intermediate in the assembly process of complete RNA polymerase, the newly described hybrid sub-assembly may function similarly as an intermediate in the formation of the hybrid form of RNA polymerase described earlier.