The assessment of the actual fatigue damage and thus the remaining lifetime of materials is a subject of enormous scientific and economical relevance. This paper describes investigations for the nondestructive characterization of microstructural changes of the metastable austenitic steel X6 CrNiTi 18 10 (AISI 321∥. Due to its good mechanical and technological properties, i.e. high toughness and corrosion resistance, AISI 321 is widely used in the chemical industry and power plant constructions. Experiments were carried out on sample batches fatigued at room temperature and T= 300°C. The nondestructive investigations and the interpretation of their results base on the knowledge of the cyclic deformation behaviour of the material. That is characterized by cyclic hardening and softening effects, which are strongly influenced by plasticity-induced martensite formation. Magnetic measurements with highly sensitive magnetometers on basis of HTC-SQUIDs yield good correlation with the actual fatigue state. The creation of martensite was found to be more significant as damage parameter than the dislocation density.