Two-dimensional crystals of the mitochondrial ATP synthase up to 0.4 microns in size were obtained from the detergent-lipid-protein micelles by detergent dialysis. A projected map of the negatively stained crystal was calculated from electron microscopical images by the Fourier-filtering procedure at ca. 2.8 nm resolution. The unit cell (with not more than two ATP synthase molecules) has the following parameters: a 13.0 nm, b 25.6 nm and gamma 86 degrees. In line with this conclusion, two alternative models for the crystal structural organization are plausible, viz., with one or two protein molecules per unit cell. The first model suggests an asymmetric incorporation of ATP synthase molecules into the lipid bilayer: extramembranous portions F1 are located on one side of the crystal membrane plane. According to the second model, the incorporation occurs on each side of the lipid bilayer, the unit cell containing the two oppositely oriented protein molecules. Based on the absence of another type of the projected crystal images (a rear view of the membrane), unique to the first model, preference is given to the second model.