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The C-terminal domain of mammalian RNA polymerase subunit IIa consists of 52-tandem repeats of the consensus sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser. This C-terminal domain is essentially unmodified in RNA polymerase IIA and extensively phosphorylated in RNA polymerase IIO. A monoclonal antibody directed against the C-terminal domain was shown by kinetic enzyme-linked immunosorbent assay to have a 10-fold higher reactivity with RNA polymerase IIA than with RNA polymerase IIO. The ability of increasing concentrations of this monoclonal antibody to inhibit the initiation and elongation phase of transcription was determined. Although both phases of the transcription reaction were inhibited, a 10-fold higher concentration of antibody was required to inhibit elongation than was required to inhibit initiation. These results support the hypothesis that RNA polymerase IIA, containing an unphosphorylated C …