A unifed framework of constructing phenomenological constitutive models for a broad class of elasto-plastic materials exhibiting either plastical incompressibility (e.g., grey cast iron) or plastical compressibility (e.g., metal foams) is proposed. The constitutive framework also enables the di.erent yielding behaviours under tension and compression as well as di.erential hardening along di.erent loading paths to be accounted for in a relatively simple manner. The resulting plasticity model does not require the diffcult task of experimentally probing the initial yield surface and its subsequent evolution; it is completely determined from a set of as few as two distinctive stress