ProteinDF is a program for performing canonical molecular orbital calculations on large proteins using density functional theory. It uses Gaussian-type orbitals and is based on a 3rd generation density functional calculation method. ProteinDF has been used to study systems such as cytochrome c with 9,600 orbitals and insulin hexamers with 26,790 orbitals. The program achieves good self-consistent field convergence through an initial guess generated by the Quasi-canonical localized orbital method.