As a prelude to a series of presentations dealing with the treatment of electron correlation and special relativity, we present the theoretical background and the implementation of a new two-component relativistic configuration interaction program. It is based on the method of generalized active spaces which has been extended from a nonrelativistic implementation to make use of two-component Hamiltonians and time reversal and double point group symmetry at both the spinor and Slater determinant level. We demonstrate how the great computational effort arising from such a general approach-the treatment of spin-orbit interaction and electron correlation in a fully variational framework-can be markedly reduced by the use of the aforementioned symmetries. Evidence for the performance of the program is given through a number of calculations on light systems with a significant spin-orbit splitting in low-lying electronic states and the well-known problem case thallium, which often serves as a rigorous test system in relativistic electronic structure calculations. (C) 2001 American Institute of Physics.