An experimental study has been made of the friction and strength properties of crystalline materials such as rock-salt, lead sulphide and ice. From frictional measurements on rock-salt and from estimates of the real area of contact during sliding, the specific shear strength of the material at the rubbing interface is calculated and found to be nearly seven times as great as the bulk shear strength of a rock-salt single crystal. It is suggested that this is because the material in the region of contact is under very high hydrostatic pressures. Independent compression experiments of rock-salt crystals confirm this. Under high hydrostatic pressures brittle fracture is prevented, marked plastic deformation occurs and the plastic yield-stress reaches values very much greater than the bulk shear strength of an uncompressed specimen. As with metals, the plastic yield stress of rock-salt may be correlated with its indentation hardness. Experiments also show that if two rock-salt specimens are pressed together between rigid anvils so that there is appreciable plastic flow, very marked adhesion occurs between the crystals and the interface is almost as strong as a single crystal. In addition, hardness measurements of the track formed during sliding show that although there is some cracking and surface fragmentation the behaviour is dominated by plastic deformation of the surface layers. These results, and similar ones obtained with other brittle materials, suggest that the frictional behaviour of brittle solids is primarily due, as with metals, to local plastic flow and to strong interfacial adhesion at the regions of real contact.