Antibodies directed against RNA polymerase II (B) from Drosophila melanogaster were obtained from rabbit sera and, as monoclonal immunoglobulins, from mouse hybridomas and shown to cross-react with the amphibian enzyme protein. Localization by indirect immunofluorescence microscopy revealed the association of this enzyme with chromatin of interphase nuclei of amphibian cells and its absence in nucleoli. Purified immunoglobulins were microinjected into nuclei of living vitellogenic oocytes of Pleurodeles waltlii and Xenopus laevis and their effects on transcriptional processes were monitored by biochemical and light and electron microscopic studies. RNA polymerase II antibodies from rabbit sera caused a rapid and almost complete release of nascent transcripts from the chromatin axis of the loops of lampbrush chromosomes, followed by collapse of the loops and their retraction on the main chromosome axis. Monoclonal murine antibodies to the large RNA polymerase II subunits also inhibited transcription in chromosome loops but appeared to inhibit initiation rather than elongation events. Activities of class I and III RNA polymerases were not significantly affected by injection of antibodies to polymerase II, indicating immunological differences between the three RNA polymerases. The potential value of the in vitro test system described, as a very sensitive assay for detecting proteins involved in transcription in living cells, is discussed.