A sophisticated apparatus has been built with which loss and magnetostriction can be measured in silicon‐iron over the frequency range 20 to 400 Hz, at inductions up to 1.9T; and under either compressive or tensile stresses up to 7×106 N/m2. Such measurements have been made on both Japanese HI‐B grain‐oriented silicon‐iron and on other commercial‐grade materials. A significant improvement in the magneto‐striction stress‐sensitivity has been observed in the newer material, and a reduction in the losses has been found (significant at 1.5%, but particularly marked at 1.7T and higher inductions). With the raising of the levels of magnetization in transformer cores, it is predicted that the new material should be considered seriously where the amortization of losses is important‐as in large distribution transformers. A reduction in noise output would be expected from transformers built using this material.