EC directive 2001/93/EC states that pigs must have permanent access to a sufficient quantity of material to enable proper investigation and manipulation activities. This directive requires further interpretation. In order to facilitate the further implementation of the directive into national, Dutch legislation a preliminary model was constructed to assess the value of different enrichment materials for pigs. Using an e-mail questionnaire expert opinion was elicited in order to 'validate' the preliminary model. In total nine senior pig welfare experts assessed 64 enrichment materials ordered according to the preliminary model. Kendall's coefficient of concordance of the experts was 0.87 (P <0.001). Materials that generated the lowest scores (on a scale from 0, low, to 10, high) included a mirror (attached to the wall), a concrete block, a rubber mat, straw at an absolute minimum, a mineral block, a heavy plastic ball, a chain, a rubber hose cross, a free toy (sow neck tether), a hanging car tire and a bucket (all median expert scores <1.5). Materials that generated the highest scores included whole straw with chopped beet roots, with maize silage or with additional feed, a bale of straw, long straw with fir branches and straw with forest bark and branches (all median expert scores > 8.0). The experts suggested a score of 5.0 as the minimum of what they considered acceptable enrichment. Materials with a median expert score of 5.0 included compost from a dispenser, straw pellets (loose or from a plastic dispenser) and straw in a metal basket. A high correlation was found between the preliminary model scores and the expert medians (0.97, P <0.05), and this finding was confirmed in a workshop at an international conference, suggesting that there may be considerable scope for modelling to support ethical and political decision-making in the area of environmental enrichment for pigs. (c) 2006 Elsevier B.V. All rights reserved.