The Kramers' restricted complete active space self-consistent-field (KR-CASSCF) method based upon two-component molecular spinors and relativistic effective core potentials including spin-orbit interactions is implemented, employing the two-step approach, in which the expansion coefficients of configurations and molecular spinors are determined alternately. The present approach allows the influence of spin-orbit interactions to be taken into account in the optimization of one-electron wave function space. Test calculations were performed for the Hg atom, Au- anion, and Tl+ cation with the closed-shell electronic configuration, the ground state potential energy curves of homo-nuclear diatomic molecules, As-2, Sb-2, and Bi-2, over a wide range of inter-nuclear distances, and the bond fission of a polyatomic molecule CH3I. The results show that the KR-CASSCF method properly describes the dissociation of molecules for the fine-structure states. It is also evident that molecular properties are affected by optimized spinors for systems containing heavy atoms such as Bi-2 and CH3I at the CASSCF level of theory. (C) 2003 American Institute of Physics.