In the literature ample attention is given to the assumptions on which the interpretation of shear-test results from a Jenike shear tester are based. Very little is said, however, about the influence on the test data of the construction of the apparatus itself. In the work described here, the authors have investigated the influence of the loading mechanism on the results. The original Jenike-type tester and a slightly modified one are considered. From the equilibrium conditions, the real normal and shear stresses on the assumed shear plane were derived, which were found to deviate from the external applied stresses. The deviations in the primary results and derived data were quantified by introducing correction factors, the boundary values of which could be calculated. Experiments with three different bulk materials were performed to measure the relevant quantities and to verify the assumptions used in theoretical analysis. Furthermore, the real normal stress on the shear plane was measured directly. From these measurements the experimental correction factors could be derived, which appeared to lie within the boundaries as theoretically predicted. In general it may be concluded that the yield loci and their derived quantities as determined by a properly used Jenike shear tester are, as a rule, suitable for design purposes of e.g. silos. For comparison of results from different testing techniques, however, the deviations introduced by the loading mechanism cannot be neglected.