Giant magnetoresistance La–Ca–Mn–O films, with 32% at 5.7 K and 20% near room temperature change in resistivity ( &Dgr;&rgr;/&rgr;0) at a magnetic field of 4.7 T, have been deposited on Si (100) substrates using pulsed laser deposition. The effects of preparation conditions on the properties of the films are described. Deposition at a high temperature shifts the resistivity peak to lower temperatures and simultaneously increases the maximum resistivity value. Annealing at a high temperature, however, produces the opposite effect on the resistivity behavior. Our results suggest that the oxygen stoichiometry in the films is a crucial factor in causing the ferromagnetic transition and the noted peak of the resistivity characteristics to shift to higher temperatures. Some possible mechanisms are suggested. ©1996 American Institute of Physics.