A huge positive magnetoresistance effect in a relatively low magnetic field (less than 0.5 T) at room temperature was discovered in MnSb granular films. A granular film consisting of nano-scale MnSb clusters was grown on a sulfur-passivated GaAs(001) substrate by molecular beam epitaxy, then covered with an Sb thin layer. The granular film shows magnetic-field-sensitive current-voltage characteristics. When a constant voltage, above the threshold value, is applied to the film, a more than 1000 percent change in the current, which we term a magnetoresistive switch, is driven by the magnetoresistance effect. A possible mechanism for the magnetoresistive switch effect is discussed.