Cast specimens of Mg and of several Mg-Zn binary alloys with a wide range of grain sizes were deformed in tension and compression. The k values calculated from the Hall–Petch (H-P) plots of the tensile 0.2 pct proof stress increased with the Zn content, from 0.24 MPa m½ for pure Mg to ~0.66 MPa m½ for the 2.3 at. pct Zn alloy; k values measured from compression tests were larger, typically by 0.05 MPa m1/2. When the strength measurements were corrected for the pseudoelastic strain resulting from elastic twinning, the k values generally increased, and the difference between tension and compression was eliminated. This showed that the larger k values obtained in compression using uncorrected data were an artifact of the pseudoelastic effect. The apparent friction stress varied between about 14 MPa for pure Mg to very low or negative values for the most dilute alloy, increasing again to about 8 MPa for the most concentrated alloy. The use of strength data corrected for pseudoelasticity effects is necessary for a consistent analysis of the grain size hardening.