Abstract. Certain thermodynamic properties of elastic-plastic materials with workhardening are discussed and their corresponding free-energy functions are determined.1. Introduction. The aim of the present paper is to examine certain thermodynamic properties of elastic-plastic materials and to determine their free-energy functions. We shall confine ourselves to considering the von Mises yield criterion, infinitesimal deformations, and isothermal conditions.It is shown in [1] how, under suitable hypotheses on the spatial gradient of velocity, the infinitesimal theory of plasticity can be deduced from the general theory of materials with elastic range, a theory formulated in [2] and [3].In order to describe the constitutive response, we use the concepts of material element, state, and process (the latter is a mapping defined on a real interval, which takes its values in the set of states and specifies the possible evolution from an initial state). We formulate these concepts as in Silhavy [4,5].The following general properties of material elements, states, and processes are important to our developments. If a material element satisfies the condition of perfect accessibility (i.e., if any two states are linked by at least one process), then the second law of thermodynamics states [6] that the work done by the exterior on the material element during a closed process is nonnegative. If the second law is satisfied, at least one state function y/ exists, called free-energy function, which satisfies the dissipation inequality, i.e., the inequality saying that the work done during any process linking two states al and a2 is not less than (t//(a2) -y/{al )). In general, the function \f/ is not unique but, for each state a , the set of all free-energy functions vanishing at a0 has least and upper elements, i.e., free-energy functions y/_ and lj7 such that ^ f°r anY free-energy function with y/{o0) -0. If a material element does not meet the condition of perfect accessibility but has a base state, i.e., a state from which every other state is reachable by some process, it is still possible to formulate the second law in such a way that its prescription is equivalent to the existence of (at least) one free-energy function [7],