It is shown that, in a mixture such as these in bone where one element is prominent because of its photoelectric effect, a realistic estimate can be made of its mass fraction despite large variations of those of the lower atomic number elements. A relationship has been derived from which the percentage of calcium by mass present can be calculated form the effective atomic number as defined and determined by the dual energy technique. Estimates of error in the experimental determination of effective atomic number Z* show that if the two effective energies are correct to +or-2 keV, with their difference correct to +or-1 keV, and with no special care in the selection of the region of interest, the calcium percentage is calculable to +or-13% for Z* values from about 10 up to 13.5. The error increases with decreasing Z* and is almost 19% at Z*=8, corresponding to a calcium percentage of 2.2+or-0.4. It is believed that these errors could be reduced by careful choice of the region of interest, e.g. by taking large trabecular areas to reduce the standard error of measurement or by selection of small cortical zones having low standard deviations of their CT numbers.