Abstract: Cubic polycrystalline bulk materials of La-Pb-Mn-O were prepared by solid state reaction method. Magnetic measurements indicated that the
samples are ferromagnetic, and T_C is 257K. Magnetotransport measurements showed that the metal-semiconductor transition temperature (T_p of
the samples is 251K, and near T_p the giant magnetoresistance (GMR) peak values reach 72% and 85% under 5T and 13T magnetic field, respectively. It was noticed that the samples present GMR effect over a wide temperature
range from 77K to room temperature. The GMR effect is related to the behavior of the spin polaron, which is affected by both temperature and applied field.
Besides the influence of the spin polaron, the GMR peak effect near T_p is also ascribed to the strong decline of spin-disorder scattering of carriers
caused by the applied field. Furthermore, the GMR effect over the whole measured temperature, especially at low temperatures far below T_p, is also
resulted from the spin-dependent scattering of carriers near grain boundary associated with the polycrystallinity of the samples. In addition, the
inhibition of the spin-polarized intergrain tunneling at T_p leads to the disappearance of the low-lield GMR effect caused by the Hund exchange.

Key words: GMR, metal - semiconductor transition temperature, polycrystalline