Nondilute athermal and theta solutions of nonadsorbing flexible macromolecules in equilibrium with repulsive slit-like pores were examined by the lattice Monte Carlo simulations. The free energy of confinement DeltaA/kT and the force f/kT exerted by polymers on the slit were computed as a function of the slit width D in a wide range of bulk concentrations phi. The free energy and force profiles in nondilute solutions were found to deviate considerably from the ideal chain theory; the perturbation of chains by a presence of the slit walls were substantially reduced in nondilute solutions. The free energy and force functions appropriate for nondilute solutions were derived by fitting the simulation data. Further, the relative pressure p(1)/p(E) exerted by the nonadsorbing confined molecules on the slit walls was calculated. The depletion effect relevant to colloid stabilization was found in dilute solutions to be slightly weaker for excluded-volume chains than for ideal chains. The relative pressure equation was modified to cover semidilute solutions, by using the mean-field and scaling expressions of the osmotic pressure. Both the relative pressure p(1)/p(E) and the intra-slit concentration profiles phi(1)(x) in tandem display a suppression of the depletion effect with increasing phi in simidilute solutions. (C) 2003 Elsevier Science Ltd. All rights reserved.