The magnetic Barkhausen noise from strain induced a'-martensite is investigated during low cycle fatigue (LCF) of 304L steel. The influence of cold-rolling prior to fatigue is also examined. Both cold rolling and LCF induce a'-martensite that is characterised by a high field Barkhausen peak, clearly distinguishable from the d ferrite peak. For the same volume fraction of a'-martensite, the Barkhausen activity is more intense for martensite induced by cold-rolling than for martensite induced by LCF. For LCF, the intensity of the peak varies tremendously along a s = f(e) loop, being enhanced in the tension part of the loop and strongly decreased in the compression part. Neither the applied stress nor the plastic strain controls the Barkhausen activity alone. A composite model is used to estimate the variation of the internal stress of type II within the martensite phase. It is concluded that the Barkhausen activity is controlled by this internal stress.