Borated stainless steel is frequently used in critical experiments for special measurements and to simulate reactor components. The validity of such measurements depends on the accuracy with which the boron density in the steel is known and on the uniformity of the boron dispersion in the steel. This work describes the technique for measuring the boron inventory in two different borated stainless steels by means of a sensitively instrumented subcritical test assembly. The results compare favorably with monochromatic neutron transmission measurements and with classical chemical analyses. The absolute calibration of the test assembly for boron was accomplished by means of steel coupons to which measured amounts of powdered elemental boron had been added.