The tensile behaviour of composites produced by infiltrating ceramic particle beds with high purity (99.99%) At is studied as a function of reinforcement size and chemistry (Al2O3 and B4C). The yield stress is higher in composites containing B4C particles, increasing with decreasing interparticle distance in both composite systems. The flow stress of the composites, when corrected for damage, displays the same dependence on interparticle distance as the yield stress. The overall strain hardening exponent, however, is independent of the microstructural scale. These observations are rationalized based on the theory of geometrically necessary dislocations. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.