This article examines the effect of the presence of a general microcrack upon the stress intensity factor al a main crack K-MA in austenitic stainless steels which had been hydrogenated and outgassed. Specifically, it is desired to obtain the different contour levels corresponding to the regions where the stress intensity factors K-I, K-II and K-III are reduced (shielding) or increased (amplification) due to the presence of microcracks in the vicinity of a main crack. The results of the work reveal that depending upon the microcrack size, location and orientation, shielding or amplification effects may became prevalent. In the case of shielding this provides a desirable toughening effect, while in the case of amplification it provides an undesirable weakening effect of the hydrogenated and outgassed austenitic stainless steels. The work provides a useful quantitative design tool and a valuable insight into the safety of these steels. (C) 1997 International Association for Hydrogen Energy.