The crystal structure of baboon alpha-lactalbumin has been determined at 6 A and at 4.5 A (0.6 nm and 0.45 nm) resolution by the method of isomorphous replacement. The principal derivative was prepared by reducing a disulphide bridge in the crystals and inserting a mercury atom. Detailed comparison of the electron-density maps with corresponding maps of hen egg-white lysozyme shows that they are closely similar, with correlation coefficients of 0.57 and 0.44 at 6 A and 4.5 A resolution respec...