In recent years, in the field of geotechnical design, software based on FEM has come to the front. Advanced computer programs make it possible to use more developed soil models besides the most current elastic-plastic Mohr-Coulomb model. By using these computer programs, nonlinear behaviour of the soil can be followed better, even in the case of more complicated load events. Among the constitutive models incorporated in the commercial software the Hardening Soil Model (HS) and Soft Soil Model (SS) are the most promising ones. Observations and experience gained in tunnel construction, deep open excavation or preloaded embankment prove that with these soil models reality can be followed more accurately, especially in cases where unloading and reloading are present. In order to produce the software input parameters, in case of more complicated soil model, more demanding laboratory tests are needed. In case of HS model one of the basic demands is to determine the power for stress-level dependency of stiffness (m), tangent stiffness for primary oedometer loading (E oed ^ref) and unloading/reloading stiffness (E ur ^ref). The paper focuses on the behaviour of transdanubian clay common in Hungary. Sampling, laboratory investigations and evaluation aimed to determine the input parameters for the HS are described. Results based on a number of oedometric tests accomplished with unloading and reloading proved to be adequate for the computational purposes. © 2009 IOS Press.