The generalized relativistic effective core potential (GRECP) approach is employed in the framework of multireference single- and double-excitation configuration interaction (MRD-CI) method to calculate the spin-orbit splitting in the ²P⁰ ground state of the Tl atom and spectroscopic constants for the 0⁺ ground state of TlH. The 21-electron GRECP for Tl is used, and the outer core 5s and 5p pseudospinors are frozen with the help of the level shift technique. The spin-orbit selection scheme with respect to relativistic multireference states and the corresponding code are developed and applied in the calculations. In this procedure both correlation and spin-orbit interactions are taken into account. A [4, 4, 4, 3, 2] basis set is optimized for the Tl atom and employed in the TlH calculations. Very good agreement is found for the equilibrium distance, vibrational frequency, and dissociation energy of the TlH ground state (R_e = 1.870 angstroms, ω-e$/ = 1420 cm^-1, D_e = 2.049 eV) as compared with the experimental data (R_e = 1.872 angstroms, ω-e$/ = 1391 cm^-1, D_e = 2.06 eV).