The load relaxation test was used to evaluate the elevated temperature (>0.4Tm) flow strength (as a function of strain rate and temperature) of a series of modified type AISI 316, austenitic stainless steels which had been given various thermomechanical pretreatments. Two classes of steels were investigated. The first contained only stabilizing additions; the second contained both stabilizing additions and copper. Under the same conditions the latter class of steels was found to have a much higher flow strength at all strain rates compared to the former class. Microstructural examinations showed that the high flow strength of the copper containing steels results from a larger number of finer matrix MC-type carbides which effectively pin dislocations. It is suggested that copper causes this finer distribution by increasing the stacking fault energy of the austenite lattice and thereby enhancing the nucleation rate of MC-type carbides on grain matrix dislocations.