In 1998 the Reyntjens Laboratory acquired a big size triaxial cell testing device for the testing of heterogeneous materials. The goal of the triaxial test set-up is to gather information about the material behaviour under a multi-axial stress state. In that it simulates the confining pressure of the surrounding material. In 1998 and 1999 the test set-up was developed. Meanwhile the first test results on the triaxial behaviour of lime mortar and masonry have been acquired. In the case of masonry cores, special attention is paid to the resulting biaxial yield-criterion. as this is of major importance for an assessment of the structural safety of the masonry. The study on lime mortar aims at determining its deformability in the case of a triaxial compression state. Especially the influence of the pore structure is evaluated. The research goal is the better understanding of the mechanical behaviour of ancient masonry, seeking for scientific explanation about the relative high mechanical strength of masonry built up with lime mortar that is considered to be a weak material. The test set-up is illustrated, the first test results and the resulting biaxial yield criterion for masonry based on cores are presented. Triaxial tests on both hydrated and hydraulic lime mortars as well as the mutual influence between the triaxial behaviour of the mortar and its pore structure are studied and discussed.