TFIID is a multiprotein complex that plays a central role in the initiation and regulation of class II transcription. Transcription factor IID (TFIID) nucleates transcription initiation complex formation by direct core promoter binding and mediates the action of transcriptional activators, in part via direct interactions with them. Molecular studies of the TFIID complex have identified multiple subunits whose potential interactions can be recapitulated in vitro with recombinant polypeptides. Here we report the cloning of human TATA box binding protein (TBP)-associated factor 20 (TAF20) and the consequent identification of an additional, related TFIID subunit, human TAF15 (hTAF15). Multiple TAF20/15 interactions have been detected within native TFIID preparations and further analyzed with recombinant subunits. Along with the demonstration of a high affinity association between TAF20/15 and TBP, the present results suggest that hTAF20/15 may complement hTAF250 in directing the association of TAFs with TBP to form a TFIID complex. Finally, we present detailed mutagenesis studies that reveal multiple, distinct interaction surfaces on the presumed globular domain of hTAF20/15 and may be used, in conjunction with structural data, to model the architecture of the TFIID multiprotein complex.