Open-pore 99.99% pure Al and Al-5 wt.% Mg foam produced by replication, with pores either 400 or 75 gm in average diameter, are tested in compression with repeated relaxation cycles, at room temperature. Similar data are collected on pure aluminium. Estimating the average in situ flow stress of the metal within the foam using the variational estimate, a plasticity size effect is evidenced in 75 gm pore foams. Activation areas are, in the 400 gm foam, similar to those of the dense constituent metal for both 99.99% pure Al and Al-5 wt.% Mg. Plotting the inverse of the activation area vs. the estimated average in situ flow stress of the metal within the foam shows that, in the finer, 75 gm pore, foam, for both 99.99% pure Al or Al-5 wt.% Mg, the work done by the applied stress in the thermally activated crossing of barriers to slip is halved. A simple scenario explaining the effect is that, along the surface of metals, image forces effectively halve the actual size of obstacles opposed to slip by forest hardening. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.