Spherical microindentation tests were performed on samples cut from an AM60B magnesium alloy die casting to determine the effects of grain size on the local flow stress. Five samples were indented in the skin region (finer grain sizes), two samples in the core region (larger grain sizes and dendrites), and one sample was indented in both the skin and core region of the die-casting. It was determined that the Hall-Petch equation is applicable for predicting the initial yield point and the flow stress at several levels of plastic strain only in the skin region, and not the core region, of the die-casting. The Hall-Petch slope and intercept stress, determined from spherical indentation in the skin region, compare accurately with previously published results. Possible reasons for indentation results from the core region deviating from the Hall-Petch relationship are discussed. The Hall-Petch slope follows a linearly increasing relation with the square root of plastic strain; however, no conclusions can be drawn concerning this because of the small range of plastic strain tested. (c) 2005 Springer Science+ Business Media, Inc.