AbstractImmunological cross‐reaction was employed for identification of proteolytic fragments ofE. coliRNA polymerase genered both in vitro and in vivo. Several species of partially denatured but assembled RNA polymerase were isolated, which were composed of fragments of the two large subunits, β and β′, and the two small and intact subunits, α and σ. Comparison of the rate and pathway of proteolytic cleavage in vitro of unassembled subunits, subassemblies, and intact enzymes indicated that the susceptibility of RNA polymerase subunits to proteolytic degradation was dependent on the assembly state.Using this method, degradation in vivo was found for some, but not all, of the amber fragments of β subunit in merodiploid cells carrying both wild‐type and mutantrpoBgenes. Although the RNA polymerase is a metabolically stable component in exponentially growing cells ofE. coli, degradation of the full‐sized subunits was found in two cases, i.e., several temperature‐sensitiveE. colimutants with a defect in the assembly of RNA polymerase and the stationary‐phase cells of a wild‐typeE. coli. The in vivo degradation of RNA polymerase was indicated to be initiated by alteration of th