The molecular structure of RNA polymerases from Escherichia coli, Salmonella typhimurium, Salmonella anatum,serratia marcescens, Aerobacter aerogens, Proteus mirabilis and Bacillus subtilis were compared based on:i) inhibition of the enzyme activity by treatment with antibodies against E. coli RNA polymerase subunits;ii) analysis of antibody precipitates by sodium ododecyl sulfatepolyacrylamide gel electrophoresis; and iii) analysis of antibody precipitates by urea-isoelectrofocusing followed by sodium dodecyl sulfate-slab gel electrophoresis in the second dimension. All the bacterial RNA polymerases examined cross-react equally with anti-E. COLI HOLOPOLYMERASE BUT EXHIbit different extents of cross-reaction with antibodies against individual subunits. Except for B. subtilis RNA polymerase, the molecular weight and isoelectric point of the enzyme subunits are close to those of E. coli polymerase. However, minor difference were found at least within the resolution of the techniques employed:S. anatum polymerase has sigma subunit larger than E. coli sigma subunit; P. mirabilis enzyme has sigma subunit larger in size and more acidic in charge, and alpha subunit smaller and more basic than corresponding E. coli subunits. The electrophoretic map of B. subtilis enzyme subunits is completely different from that of E. coli enzyme.