The Drosophila morphogenetic protein Bicoid (Bcd) is required for the development of anterior structures of the embryo. Bcd, a homeodomain protein, is distributed as an anterior-to-posterior gradient in the embryo. It stimulates the expression of the hunchback (hb) gene in the anterior half in an all-or-none fashion. We have recently shown that Bcd binds cooperatively to a hb enhancer element and proposed that cooperative DNA binding is facilitated by an interaction between Bcd molecules. In this report, we further analyze the interaction between Bcd molecules and define regions important for protein-protein interaction. We show that the homeodomain of Bcd alone fails to interact with another Bcd molecule efficiently. The protein sequence flanking either side of the homeodomain restores the protein-protein interaction function. Mutations in the homeodomain that affect DNA binding do not adversely affect the protein-protein interaction function, suggesting that the surfaces for DNA binding and protein-protein interaction are separable. Finally, we demonstrate that the homeodomain of Bcd alone, unlike the intact Bcd, fails to bind DNA cooperatively. These results further support the notion that cooperative DNA binding is facilitated by the interaction between Bcd molecules. They strongly suggest that protein-protein interaction is an important property of Bcd for its biological activities.