Determinations were made of the specific gravity and of the calcium, phosphorus, nitrogen and water content of specimens of human cortical bone. The results were found to show small but consistent variations with the ages of the subjects. A type specimen was therefore constructed from the averages of the observed figures from the bones of four young adult men. Values for oxygen, hydrogen and carbon were derived from the amounts of compounds of these elements which were known to be present. The amounts of sodium, magnesium and sulfur were obtained from the literature. A table of the average elementary composition of adult human cortical bone was prepared from this material. This table is suitable for use in computing the absorption of ionizing radiation by this substance. Problems in the computation of energy absorption in spongiosa are discussed.