We investigated the role of TFIIH in transcription by RNA polymerase II (pol II) in vivo by microinjection of antibodies against this factor into Xenopus oocytes. Five different antibodies directed against four subunits of TFIIH were tested for effects on transcription of coinjected human immunodeficiency virus type 2 and c-myc templates. Each of these antibodies severely reduced the efficiency of elongation through human immunodeficiency virus type 2 and c-myc terminator elements. In contrast, an anti-TFIIB antibody did not inhibit elongation. Anti-TFIIH antibodies also had a much smaller inhibitory effect on total transcription than did anti-TFIIB or anti-pol II large subunit. Three inhibitors of TFIIH kinase activity, H-7, H-8, and dichlororibofuranosylbenzimidazole (DRB), inhibited elongation similarly to anti-TFIIH antibodies. These results strongly suggest a role for TFIIH in the stimulation of transcriptional elongation in vivo.