ESDU Struct 01.01.19 introduces two methods for calculating the elasto-plastic local buckling stress and crippling stress of rectangular tube section struts. The first method is provided in the form of a graph giving the elastic buckling stress coefficient as a function of the ratios of the cross-sectional dimensions of the tube (the ratio of the widths of the sides of the tube and the ratio of the thicknesses of the sides of the tube) together with an equation giving the crippling stress in terms of the 0.2 per cent proof stress and the buckling stress. An example of estimating the elasto-plastic buckling stress through use of the material tangent modulus is given. The second method is provided in the form of a Fortran program (ESDUpac S1119@ available as source code and in compiled form) which uses the Rayleigh-Ritz method to determine the elasto-plastic local buckling stress from the buckling condition for the tube. A second empirical relationship is used to estimate the crippling stress. The elasto-plastic stress-strain curve for the tube material may be modelled using either the Ramberg-Osgood model or the equivalent@ but differently represented@ model described in ESDU 76016. The construction of the input to the program and the format of the output are explained and illustrated with a worked example. If desired@ the program will output buckling stresses for a range of buckle wavelengths. Warning and error messages are explained. The program is provided in a form that prompts on-screen for input data.