The paper presents a nondestructive testing technique developed by ISMES to determine the mechanical characteristics of masonry structures by means of thin flat-jacks inserted into the mortar layers. In a first phase the existing state of pressure in a certain point of the masonry is determined by using the pressure release technique and the subsequent reloading by means of one flat-jack. In the second phase another jack is inserted, parallel to the first one, and a uniaxial compression test is carried out on the masonry specimen interposed between the two jacks. In this way it is possible to determine the deformability modulus and the compressive strength of the masonry. The reliability of the testing technique has been fully investigated by means of calibration tests carried out in a laboratory on large-size masonry walls. Three examples of application of flat-jacks tests are presented. Two of them are related to brick masonry structures (St. Eustorgio Cloister and the wall of Leonardo's Last Supper -- Milan) and the third is related to a stone-masonry building (Madonna del Prato Church -- Gubbio).