The HIV-1 trans-activator Tat increases the rate of transcription from the HIV-1 LTR promoter through the stem-loop-containing TAR RNA. To analyze the mechanisms of Tat action, a cell-free trans-activation system with no preincubation has been developed. Recombinant Tat specifically increased the level of a long runoff transcript but not a promoter-proximal transcript in a TAR-dependent fashion. These observations and the result of pulse-chase experiments support strongly the hypothesis that Tat enhances the ability of RNA polymerase to elongate over longer distances. Increased levels of the purified cellular factor TFIIF, essential for initiation and also implicated in elongation of transcription, obviated trans-activation by Tat by increasing the basal (Tat-independent) activity. However, another elongation factor, ATN/TFIIS, showed synergistic activation with Tat. An antiserum against a recombinant form of the large subunit of TFIIF (RAP 74) preferentially suppressed the activated level of transcription exerted by Tat. We propose the hypothesis that Tat acts as a processivity factor on RNA polymerase II in an analogous manner to TFIIF.