Mechanical properties of high strength steel in the plastic range are of fundamental importance especially when it is applied to the plastically designed structures. The structure may fail because. of various types of buckling : column buckling of the bending type, torsional or local buckling. Often the failure is due to a combination of these different types of buckling. In order to be able to solve these problems theoretically it is necessary to know the relationship between increments of stress and strain after the material has been loaded and deformed up to a certain state and along a prescribed loading path. For the case of flange buckling of wide flange sections the resistance against buckling depends. almost completely on the relationship between additional shear-stresses and strains following a given amonnt of axial compression. The tests described in this report were an sttempt to measure the tangent shear modulus G _t directly. Round bar specimens machined from the plates of high strength steel were axially loaded up to the strain-hardening range. Keeping the axial load constant, the torsional moments were applied next. By means of a specially made foil gage for the measurement of plastic strain both axial and torsional strains were measured. Initially the value of G_t approached the elastic value G =8, 000 kg/mm²but dropped rapidly with_ increasing values of shear strain γ. For γ=1. 02. 0× 10^-3 it is found that G _t=2, 5001, 500 kg/ mm². From there on G _t decreased slowly with increasing γ, and for γ=5× 10^-3, G _t=1, 000 kg/mm². Thus it can be concluded that the flow theory of plasticity which predicts G _t= G (elastic) is true, but only for very small shear strains. Due to unavoidable initial imperfections the initial. behavior for small shear strains cannot be of determing influence. The above finding value of G _t ≈2, 000kg/mm² may be used for the prediction of lateral-torsional buckling of high strength steel structures. The theoretical analysis on G _tbased on the flow theory of plasticity was confirmed by the test results.