Compression testing of cellular sandwich core materials is often complicated by the presence of edge effects, in which the cells on the outside of the specimen deform at lower stresses than those in the bulk. As a result, minimum specimen length-to-cell-size ratios have been established in the literature for metallic foams. Periodic cellular metals (PCMs) are a recently developed class of materials that have received considerable interest due to their greater architectural efficiency and enhanced mechanical properties compared to conventional metal foams. In this study, edge effects of PCM core materials have been measured in compression using a new test platform which evaluates the properties of the core as a stand-alone material. This approach introduces uniform and periodically rigid boundary conditions to the PCM nodes, resulting in the same inelastic buckling failure mechanism that occurs in reference PCM sandwich panels. Edge effects were comparatively small and samples down to 2 x 2 unit cells were found to be representative of bulk-like cores.