Resistance measurements indicate the presence of magnetoresistance in the La0.5Sr0.5MnO2.5 brownmillerite related compound. An 80% of magnetoresistance is found at 75 K. In spite of the partial break-up occurring at the 3D network of octahedra sharing corners, characteristic of the full oxygen content perovskite phase, the oxygen deficient compound exhibits complex magnetic and electric properties. Such behavior can be explained on the basis of ferromagnetic and metallic clusters randomly distributed at the octahedral layers separated from each other by an insulating antiferromagnetic matrix. AC susceptibility measurements suggest spin glass behavior at low temperature as a consequence of the competition between different magnetic interactions.