Lattice Monte Carlo simulations have been employed to calculate depletion interaction of excluded volume chains in a weakly attractive slit, particularly in the region around the critical point of adsorption. The simulations were performed under full equilibrium conditions where a dilute solution in a slit was in contact with the reservoir. The free energy of confinement DeltaA, the force f, and the relative pressure p(I)/p(E) on the slit walls were calculated as a function of slit width D and the attraction strength epsilon. The depletion region in the pressure profile p(I)/p(E) vs D is reduced by an increase in the attraction potential epsilon in a manner resembling the influence of polymer concentration. At the critical point of adsorption epsilon(c) the depletion interaction vanishes both in the pressure p(I)/p(E) and in the intraslit concentration profile phi(I)(x). The parameters used to assess the stability of colloidal dispersions such as the depletion potential W(D) (an integral of the net pressure Delta(P)) reach a unique value at the critical condition. A monotonic repulsive profile p(I) vs D was found for chains trapped in the slit at restricted equilibrium. The mean dimensions of chains compressed in attractive slits feature a distinct minimum at intermediate slit widths.