The androgen receptor (AR) is a member of the nuclear hormone receptor superfamily. Recent work in this field has been focused upon defining the mechanisms of transcriptional control exacted by members of this superfamily. Using a COOH-terminal region of the human AR in a yeast two-hybrid screen, we have identified Tip60 as an AR-interacting protein. In this report, we show that Tip60, which was originally identified as a coactivator for the human immunodeficiency virus TAT protein, can enhance AR-mediated transactivation in a ligand-dependent manner in LNCaP and COS-1 cell lines. In addition, our experiments show that Tip60 can also enhance transactivation through the estrogen receptor and progesterone receptor in a ligand-dependent manner; thus identifying Tip60 as a nuclear hormone receptor coactivator. Our studies also demonstrate that Tip60 co-immunoprecipitates with the full-length AR in vitro and that, in our system, Tip60 enhances transactivation to levels observed with the coactivators steroid receptor coactivator 1, p300, and CREB-binding protein. The importance of such proteins in enhancing nuclear hormone receptor-mediated transcriptional activation is widely accepted, and this work suggests that Tip60 may have an equally important role to play.