We present magnetic and electric transport studies on several series of La0.7Ca0.3MnO3 polycrystalline pellets and thin films. The thin films were obtained by DC magnetron sputtering on several substrates and by sol-gel deposition technique on Si (100). The diffraction patterns show the polycrystalline character of films grown on Si by both techniques. The morphology was studied by AFM and the mean grain sizes were obtained. The steady decrease of the low-temperature magnetoresistance (MR) at high fields is explained by the increase of the effective section for conduction by the orientation of pinned Mn magnetic moments at the surface of the grains and, therefore, depend strongly on the connectivity between grains. On the contrary, the low-field MR is related to the magnetic behavior of the ferromagnetic domains of the grains. Low- and high-field MR characteristics of thin films deposited on different substrates are correlated to their microstructure. (C) 2000 Elsevier Science S.A. All rights reserved.