To understand electronic properties and reactivities of proteins, we developed a molecular orbital (MO) program ProteinDF based on the density functional theory with Gaussian functions. By using it, we recently performed an all-electron calculation on a typical hemoprotein, cytochrome c. ProteinDF was coded by the object-oriented language C++ to construct the complex MO program for calculations on proteins with over a thousand orbitals. This program was parallelized for distributed parallel processing systems such as workstation clusters. In this Letter, we report parallelization methods by using the object-oriented technology in ProteinDF and their efficiency. (C) 2001 Elsevier Science B.V. All rights reserved.