Transcriptional activation by CREB and CREM requires phosphorylation of a serine residue within the activation domain (Ser 133 in CREB; Ser 117 in CREM) which as a result interacts with the coactivator CBP. The activator CREM is highly expressed in male germ cells and is required for post-meiotic gene expression. Using a two-hybrid screen, we have isolated a testis-derived complementary DNA encoding a protein that we term ACT (for activator of CREM in testis), a LIM-only protein which specifically associates with CREM. ACT is expressed coordinately with CREM in a tissue- and developmentally regulated manner. It strongly stimulates CREM transcriptional activity in yeast and mammalian cells and contains an intrinsic activation function. As ACT bypasses the classical requirements for activation, namely phosphorylation of Ser 117 and interaction with CBP, it represents a new route for transcriptional activation by CREM and CREB. ACT may define a previously undiscovered class of tissue-specific coactivators whose function could be specific for distinct cellular differentiation programmes.