The Elongin (SIII) complex activates elongation by mammalian RNA polymerase II by suppressing transient pausing of the polymerase at many sites within transcription units. Elongin is a heterotrimer composed of A, B, and C subunits of 110, 18, and 15 kilodaltons, respectively. Here, the mammalian Elongin A gene was isolated and expressed, and the Elongin (SIII) complex reconstituted with recombinant subunits. Elongin A is shown to function as the transcriptionally active component of Elongin (SIII) and Elongin B and C as regulatory subunits. Whereas Elongin C assembles with Elongin A to form an AC complex with increased specific activity, Elongin B, a member of the ubiquitin-homology gene family, appears to serve a chaperone-like function, facilitating assembly and enhancing stability of the Elongin (SIII) complex.