  Measurements of magnetoresistance and magnetization were carried out on
ceramic samples of La_{0.5}Pb_{0.5}MnO_{3} and La_{0.5}Pb_{0.5}MnO_{3},
containing 10 at. % Ag in a dispersed form. The results obtained for the
resistivity at zero applied magnetic field exhibit a shallow minimum at
temperatures T about 25 K - 30 K, which shifts towards lower temperatures upon
applying magnetic field and disappears at a certain (rather low) field. Also
the resistivity at helium temperature decreases upon applying magnetic fields.
It is shown that the model of spin-dependent carrier tunneling between
antiferromagnetically coupled grains may account for the results observed.
