In this article, we show how colossal magnetoresistance effect (CMR) can be tuned in polycrystalline mixed valence manganite La _0.67 Ca _0.33 MnO ₃ via changing grain size by means of a sol-gel method. Below a critical diameter (150 nm), CMR disappears, but large intergrain MR remains even well above T_c (1.2T_c for ≈95 nm particles). Possible explanation for this effect involves single magnetic domain behavior in samples annealed at low temperature. © 1999 American Institute of Physics.