The paper describes a new method for the non-destructive measurement of the mechanical hardness of ferromagnetic components, based on the relationship between a “coefficient of magnetic hardness” and the variables in the upper portion of the magnetization curve. It is shown that there is a direct relationship between the magnetic permeability and the mechanical hardness of a low-alloy-content steel, and this is used as the basis of a d.c. magnetic method of hardness measurement for two steel components of diverse character. The construction and performance of portable apparatus for the testing of armour-piercing shot and crank-end bushes is described in detail, and the test results obtained are discussed. The reproducibility of readings is better than 1%, almost linear correlation between instrument reading and mechanical hardness is obtained, and the sensitivity is of a reasonable order. The simplicity of operation is such that unskilled personnel may be employed for routine testing.