Transcription elongation factor S-II was originally purified as a specific stimulator of transcription by RNA polymerase II. Recent studies suggest that S-II participates in gene-specific transcriptional activation in vivo, despite the fact that it directly binds RNA polymerase II and does not recognize specific DNA sequences. In this study, under the hypothesis that S-II requires co-factors to regulate the expression of specific-genes in vivo, we searched for factors that directly interact with S-II using a yeast two-hybrid system, and isolated a novel nuclear protein, FESTA. FESTA is expressed specifically in kidney and spleen, supporting our notion that S-II participates in gene-specific regulation. Two mRNA isoforms of FESTA encoding proteins with different sizes were identified and named FESTA-S and FESTA-L. FESTA contains a serine-rich region and a C-terminal tail that are highly similar to those of the ELL-associated factor EAF1. Reporter gene assays indicated that both GAL4-FESTA-S and GAL4-FESTA-L fusion proteins have trans-activating ability. Furthermore, deletion of the C-terminal tail of FESTA dramatically reduced its trans-activating ability and abolished its interaction with S-II. This study is the first report of a transcriptional activator that directly interacts with S-II and contains a transcriptional activation domain that cooperates with S-II via direct interaction.