The ATP synthase is an oligomeric complex central to cell bioenergetics. In mammals, it is responsible of the production of ~95 percent of the ATP. The bovine ATP synthase is formed from at least 15-17 different subunits and is putatively associated with the adenine nucleotide carrier and the phosphate carrier. The three dimensional structure of the hydrophilic segment of the ATP synthase (the F1) is known at medium resolution through x-ray crystallography. As is the general case for membrane proteins, the structure of the membrane spanning segment (Fo) still is unknown. To further our understanding of the structure and mechanism of the ATP synthase we have purified the enzyme from beef heart and used it to obtained 2D crystals. During the purification of the ATP synthase care was taken to preserve the protein complex in its native state, as evidenced by its ATP hydrolysis activity, as well as its sensitivity to specific inhibitors. We have obtained good results using several different detergents, such as digitonin, CHAPS, cholate and others. As a by-product, we have also purified the adenine nucleotide carrier. 2D crystallization is an alternative to the 3D crystals commonly used for x-ray crystallography and it is well suited for membrane proteins since it provides a native-like membrane environment. The factors that determine crystal size and quality are lipid to protein ratio, the type of lipid used, the detergent used for the protein preparation, temperature, etc. Currently, we are setting trials to obtain high quality 2d crystals of the ATP synthase as well as the adenine nucleotide carrier. 2D crystallization is a powerful tool for structure determination of membrane proteins.