MgB2 polycrystalline samples were fabricated under varying conditions of isostatic pressing in argon gas. The critical current densities (JC) were determined through measurements of hysteresis loops, and the highest value of JC at 10 K was 1.9 × 104 A/cm2 at 4.8 T. The depinning temperatures were measured at various magnetic fields using the vibrating reed technique. Flux jumps appeared below 7.4 K. The hysteresis loops were carefully examined to determine the temperature and magnetic field range where flux jumps appeared. © 2005 Elsevier Ltd. All rights reserved.