Control elements of many genes are regulated by multiple activators working in concert to confer the maximal level of expression, but the mechanism of such synergy is not completely understood. The promoter of the human macrophage colony-stimulating factor (M-CSF) receptor presents an excellent model with which we can study synergistic, tissue-specific activation for two reasons. First, myeloid-specific expression of the M-CSF receptor is regulated transcriptionally by three factors which are crucial for normal hematopoiesis: PU. 1, AML1, and C/EBPα. Second, these proteins interact in such a way as to demonstrate at least two examples of synergistic activation. We have shown that AML1 and C/EBPα activate the M-CSF receptor promoter in a synergistic manner. As we report here, AML1 also synergizes, and interacts physically, with PU. 1. Detailed analysis of the physical and functional interaction of AML1 with PU. 1 and C/EBPα has revealed that the proteins contact one another through their DNA-binding domains and that AML1 exhibits cooperative DNA binding with C/EBPα but not with PU. 1. This difference in DNA-binding abilities may explain, in part, the differences observed in synergistic activation. Furthermore, the activation domains of all three factors are required for synergistic activation, and the region of AML1 required for synergy with PU. 1 is distinct from that required for synergy with C/EBPα. These observations present the possibility that synergistic activation is mediated by secondary proteins contacted through the activation domains of AML1, C/EBPα, and PU. 1.